JPH04269886A - Manufacture of semiconductor laser - Google Patents

Abstract

PURPOSE:To form a mixed crystal layer having low carrier concentration by forming a dielectric film having a partial opening on a double heterostructure and heat-treating a substrate directly under the film. CONSTITUTION:A double heterostructure in which a quantum well active layer 13 is interposed between InGaAlP clad layers 11 and 15, is formed on an n-type GaAs substrate 10. An SiO2 film 16 having a stripelike opening is formed on the structure. Then, the substrate 10 is heat-treated to form a mixed crystal directly under the film 16 of the layer 13. Then, the layer 13 is buried with a mixed crystal layer 19 having a lower refractive index and larger band gap than those of the layer 13 to form a refractive index waveguide structure.

Description

[Detailed description of the invention]

[Purpose of the invention]

0001

[Field of Industrial Application] The present invention relates to a semiconductor laser used as a light source for optical information processing, optical measurement, etc.
The present invention relates to a method for manufacturing a semiconductor laser using nGaAlP-based materials.

0002

[Prior art] In recent years, I
Red semiconductor lasers using nGaAlP-based materials have been commercialized and are expected to be used as light sources for high-density optical disk devices, laser beam printers, barcode readers, optical measurements, etc. For such applications, it is necessary to focus the laser beam into a minute spot, and it is important to have stable transverse mode oscillation and a small astigmatism difference between the laser beams.

In order to achieve the above characteristics, it is necessary to use a refractive index guided transverse mode control semiconductor laser. Furthermore, forming the active layer into a quantum well (QW) structure is effective in lowering the threshold voltage and shortening the wavelength of a semiconductor laser. Therefore, a transverse mode controlled quantum well laser is strongly desired.

As this type of semiconductor laser, there is a buried type semiconductor laser in which the active layer is buried with a layer having a larger band gap and lower refractive index than the active layer. To manufacture this laser, first, as shown in FIG. 8(a), a double heterostructure in which the active layer 83 is a multiple quantum well (MQW) is formed on a compound semiconductor substrate 80, and then a SiO2 film 86 is formed.
are formed into stripes. Here, optical guide layers 82 and 84 are inserted between the active layer 83 and the cladding layers 81 and 85, respectively.

Next, as shown in FIG. 8(b), SiO
2 Active layer 83, leaving the area masked by film 86
A mixed crystal layer 89 is formed by diffusing Zn into the MQW structure. This mixed crystal layer 89 has a larger band gap and a smaller refractive index than the active layer 83. Therefore, SiO2
The striped active layer 83 at the bottom of the film 86 is buried on both sides with a mixed crystal layer 89, thereby forming a refractive index guided type buried semiconductor laser.

However, this type of method has the following problems. That is, since the mixed crystal layer formed by Zn diffusion contains a large amount of Zn, the carrier concentration of the mixed crystal layer is very high, about 1019 cm-3. For this reason, a considerable amount of current also flows into the mixed crystal layer, making it difficult to efficiently inject current into the active layer embedded in a striped pattern. Furthermore, if a window structure laser is created using a mixed crystal layer formed by Zn diffusion as a window on the laser end face, current will also flow through the window. For this reason, it is not possible to create a non-injection type window structure laser using conventional methods.

Therefore, for these reasons, in conventional buried semiconductor lasers fabricated using impurity diffusion, a large current flows through the mixed crystal layer, which is the buried layer, and the oscillation threshold of the device decreases. it was high. Furthermore, when a window structure laser is created, the operating current becomes large because current flows into the window, making high-output operation difficult.

[0008]

[Problems to be Solved by the Invention] As described above, in the conventional method of forming a mixed crystal layer by impurity diffusion, the carrier concentration of the mixed crystal layer becomes high, and it is difficult to use this mixed crystal layer as a buried layer or a window structure. There was a problem that the characteristics of the laser used were deteriorated.

The present invention has been made in consideration of the above circumstances, and its purpose is to be able to form a mixed crystal layer with a low carrier concentration, thereby reducing the oscillation threshold and increasing the output. It is an object of the present invention to provide a method for manufacturing a semiconductor laser that can contribute to the above. [Structure of the invention]

0010

[Means for Solving the Problems] The gist of the present invention is to heat-process the buried layer of a buried semiconductor laser and the window portion of a window structure semiconductor laser, instead of using a mixed crystal layer mixed by impurity diffusion. The purpose is to use a mixed crystal layer in which the active layer is mixed crystal.

That is, the present invention is a method for manufacturing a semiconductor laser having a buried type or a window structure, in which a double heterostructure having an active layer sandwiched between cladding layers is formed on a compound semiconductor substrate, and then the double heterostructure is A dielectric film having a partial opening thereon is formed, and then the substrate is heat-treated to make the part of the active layer immediately below the dielectric film into a mixed crystal, thereby making the active layer more dense than the active layer. In this method, a refractive index waveguide structure is formed by embedding a mixed crystal layer with a low refractive index and a large band gap.

Here, if the openings in the dielectric film are formed into stripes, a buried semiconductor laser can be obtained in which the active layer is formed in stripes and both sides of the active layer are filled with mixed crystal layers. In addition, if a dielectric film is formed in stripes in a direction perpendicular to the laser waveguide direction and cleaved along the stripe direction after heat treatment, a window-structured semiconductor with the cavity end face buried in a mixed crystal layer can be created. Laser is obtained. Furthermore, if the opening of the dielectric film is made into a strip, a window structure buried semiconductor laser can be obtained in which both side surfaces and both end surfaces of the strip-shaped active layer are buried with a mixed crystal layer. Moreover, the following are mentioned as desirable embodiments of the present invention. (1) Forming the active layer into a single or multiple quantum well structure. In addition to this, the quantum well active layer is sandwiched between light guide layers. (2) SiO2 or Si3 N4 as dielectric film
be used.

(3) As a heat treatment method, 750-9
Anneal at 50°C for 1 hour or more, and perform rapid thermal annealing for about 15 seconds at least once. Further, as the atmosphere for the heat treatment, an atmosphere during crystal growth, for example, an H2 atmosphere may be used. (4) Use InGaAlP as a compound semiconductor material. (5) Use metal organic vapor phase epitaxy as the crystal growth method. (6) If the surface of the compound semiconductor substrate is from the (100) plane
The surface must be inclined in the range of 5° to 40° toward one of the 011> directions.

[0014]

[Operation] After forming a structure including a quantum well structure in which the active layer is a single or multiple quantum well, SiO2
A film is selectively formed and then heated at a temperature of about 900°C for 1
It is known that by annealing for about an hour, since the active layer and quantum well structure are thin film layers, the layer immediately below the SiO2 film can be easily mixed with the layer adjacent to it (Autumn 1990). , 51st Academic Conference of the Japan Society of Applied Physics
Lecture proceedings p1100 26p-ZL-1). The mechanism of this phenomenon is not clear, but when there is a SiO2 film on top of the thin film layer, the atomic arrangement is disturbed, and S
This is believed to be because the atomic arrangement does not occur in the absence of the iO2 film.

In the present invention, first, a structure including a double heterostructure in which a bulk active layer is sandwiched between cladding layers or a quantum well structure in which the active layer is a single or multiple quantum well is formed on a compound semiconductor substrate, and then a buried structure is formed. A dielectric film, for example SiO2, is formed over the area where the layer or window is to be formed. After that, when annealing is performed at a temperature of about 900°C for about 1 hour by heat treatment, since the active layer and quantum well structure are thin film layers, these layers directly under the SiO2 film can be easily separated from the adjacent layers. Can be mixed crystallized.

[0016] Here, the mixed crystal part of the bulk or quantum well structure active layer has a larger band gap and a lower refractive index than the non-mixed crystal part where no dielectric film is present above. . Therefore, if the dielectric film is patterned to have, for example, stripe-shaped openings, this mixed crystal layer can be used as a buried layer and the active layer can be buried in stripes to form a refractive index guided type buried layer. Semiconductor lasers can be created. In addition, if a dielectric film is formed in a stripe shape perpendicular to the laser waveguide direction and then cleaved from above this dielectric film along the stripe direction after heat treatment, this part can be used as a window on the laser end face. As a part, a semiconductor laser with a window structure can be created. Furthermore, if the dielectric film is formed to have a rectangular opening, an embedded semiconductor laser having a window structure can be produced.

The buried layer or window portion thus obtained has a lower carrier concentration in the film since no impurity is used, compared to the conventional method in which mixed crystal formation is achieved by impurity diffusion. As a result, the current flowing into the buried layer or the window can be significantly reduced compared to the conventional method. Therefore, according to the present invention, with respect to a buried semiconductor laser, heat generation during operation can be prevented by reducing the operating current, and the temperature characteristics can be significantly improved. In addition, with regard to semiconductor lasers with a window structure, since the current flowing into the window portion is reduced, the operating current during high optical output operation can be significantly suppressed.
Operation with even higher optical output than before is possible.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be explained below with reference to illustrated embodiments.

FIG. 1 is a cross-sectional view showing the manufacturing process of a semiconductor laser according to a first embodiment of the present invention. First, Figure 1
As shown in (a), a thickness of 1.
μm n-In0.5 (Ga0.3 Al0.7)0
．． 5 P cladding layer 11, 0.1 μm thick In0.5
(Ga0.5 Al0.5 )0.5 P light guide layer 12, multiple quantum well (MQW) active layer 13, thickness 0.1
μm of In0.5 (Ga0.5 Al0.5)0.
5 P light guide layer 14, 0.5 μm thick p-In0.
A 5 (Ga0.3 Al0.7 )0.5 P cladding layer 15 is sequentially grown using, for example, MOCVD. Subsequently, on the p-cladding layer 15, a dielectric film 16, for example, a SiO2 film 16 having a thickness of 0.2 μm and having a striped opening with a width of 2 μm is formed.

Here, the MQW active layer 13 is as shown in FIG. 2(a).
As shown in 8 nm thick In0.5 Ga0.5
P well layer 13a and 4 nm thick In0.5 (Ga0.
5Al0.5)0.5P barrier layers 13b are alternately stacked, and has a structure in which four well layers 13a are each partitioned by barrier layers 13b.

Next, as shown in FIG. 1(b), SiO
2. On the p-cladding layer 15 exposed in the opening of the film 16,
p-In0.5 (Ga0.3 Al
0.7 ) 0.5 P cladding layer 17 and thickness 0.05
μm p-In0.5 Ga0.5 P current conduction facilitation layer 1
8 is selectively regrown to form a ridge stripe structure. For example, in the case of a (100) substrate, [01-1
] If a SiO2 mask is formed in stripes parallel to the direction, this ridge will have a regular mesa structure.

At this time, the MQW active layer 13 will be annealed during the regrowth of the cladding layer 17, and the MQW active layer 13 will be annealed during the regrowth of the cladding layer 17.
Since the QW active layer 13 has a thin film structure, as shown in FIG. 2(b), the MQW active layer 13 directly under the SiO2 film 16
becomes a mixed crystal and becomes a mixed crystal layer 19. As a result, the mixed crystal layer 19 becomes the MQW active layer 13 directly under the ridge stripe structure.
The band gap is larger and the refractive index is smaller than that of the MQW, so that an MQW buried structure is formed.

If the mixed crystal layer 19 is insufficiently mixed, annealing for 1 hour at 850 to 950° C. or 15 seconds in an H2 atmosphere, for example, after or before the regrowth of the p-cladding layer 17 is performed. Rapid thermal annealing (RTA)
) at least once. This allows the mixed crystal formation to proceed reliably.

Finally, as shown in FIG. 1(c), SiO
2 After removing the film 16 by etching, the thickness is 3 μm.
A p-GaAs contact layer 20 of
By forming uGe/Au, a buried semiconductor laser with a refractive index waveguide structure is completed.

In the semiconductor laser thus fabricated, there is a high heterobarrier between the p-cladding layer 15 and the contact layer 20, which acts as current confinement. Furthermore, since there is a current conduction facilitation layer 18 between the p-cladding layer 17 and the contact layer 20, the p-cladding layer 17
The heterobarrier between the contact layer 20 and the contact layer 20 is effectively lowered, making it easier for current to flow in this portion. Therefore, current is efficiently injected into the MQW active layer 13 embedded in a stripe shape. Since the mixed crystal layer 19 has a larger band gap and a lower refractive index than the buried MQW active layer 13, a buried refractive index waveguide structure is formed and stable transverse mode oscillation is achieved. Note that the thickness of the well layer 13a is 6 to 6.
The highest gain is obtained at 9 nm, which is advantageous as it enables low threshold operation.

As described above, in the method of this embodiment, the mixed crystal layer 19 for burying the quantum well active layer 13 is formed by heat treatment instead of impurity diffusion.
The carrier concentration can be lowered, and the current flowing into the mixed crystal layer 19 can be significantly reduced. Therefore, in addition to reducing the oscillation threshold, heat generation during operation can be prevented by reducing the operating current. That is, it is possible to realize an embedded semiconductor laser which has much better temperature characteristics than the conventional one and can operate up to a higher optical output than the conventional one.

FIG. 3 is a sectional view showing a schematic structure of a semiconductor laser according to a second embodiment of the present invention. Furthermore, Figure 1
The same parts are given the same reference numerals, and detailed explanation thereof will be omitted.

This embodiment differs from the first embodiment described above in that the MQW structure of the active layer 13 and the optical guide layer 1
2,14, the active layer is made of Indium with a thickness of 0.03 μm.
This is an example of a double heterostructure with a 0.5 Ga0.5 P layer 33. The creation procedure is the same as in the first example. When the thickness of the active layer 33 is as thin as 0.03 μm or less, the mixed crystal effect can be easily obtained as in the case of the QW structure. The embedded DH laser obtained here is
It operates on the same principle as the first embodiment.

FIG. 4 is a sectional view showing a schematic structure of a semiconductor laser according to a third embodiment of the present invention. Furthermore, Figure 1
The same parts are given the same reference numerals, and detailed explanation thereof will be omitted.

In this embodiment, after forming the contact layer 20 to the state shown in FIG. 1(b) in the same manner as the first embodiment, the contact layer 20 is successively formed with the p-cladding layer 17 and the conductive layer without removing the SiO2 film 16. The layer is grown on the facilitating layer 18 . Finally, Ti/Pt/Au was formed as the p-side electrode 41, and AuGe/Au was formed as the n-side electrode 22 in the same manner as in the first embodiment. The operating principle of the laser of this embodiment is basically the same as that of the first embodiment, but in this laser structure, SiO2
The membrane 16 directly performs current confinement. In this case, the first
Since the number of crystal growth and processes can be reduced compared to the embodiment described above, productivity is increased.

Note that in the configuration of FIG. 4, a double hetero structure using an InGaP active layer may be used instead of the MQW active layer 13 and the optical guide layers 12 and 14, as in the second embodiment.

FIG. 5 is a cross-sectional view showing the manufacturing process of a semiconductor laser according to a fourth embodiment of the present invention. Furthermore, Figure 1
The same parts are given the same reference numerals, and detailed explanation thereof will be omitted.

In this example, as shown in FIG. 5(a), a 1 μm thick n-In0
．． 5 (Ga0.3 Al0.7 )0.5 P cladding layer 11, 0.1 μm thick In0.5 (Ga0.5
Al0.5)0.5P optical guide layer 12, multiple quantum well (MQW) active layer 13, 0.1 μm thick In0.
5 (Ga0.5 Al0.5 )0.5 P light guide layer 14, 1 μm thick p-In0.5 (Ga0.3
Al0.7)0.5 P cladding layer 15, thickness 0.0
A 5 μm thick p-In0.5 Ga0.5 P conduction facilitation layer 18 and a p-GaAs contact layer 20 are made of, for example, MO.
They are sequentially grown and formed using the CVD method. After that, contact layer 2
0 with a stripe-like opening 2 μm wide on the 0
．． A 2 μm thick SiO2 film 16 is formed.

Next, as in the first example, annealing was performed at 850 to 950° C. for 1 hour in an H2 atmosphere, or
By performing rapid thermal annealing (RTA) for about 5 seconds at least once, the SiO
2. The MQW active layer 13 directly under the film 16 is made into a mixed crystal. Subsequently, the p-side electrode 41 was formed of Ti/Pt/Au, and the n-side electrode 22 was formed of AuGe/Au.

In this embodiment, the SiO2 film 16 serves as a current blocking layer, and current can be injected only into the striped active layer 13. Further, since the mixed crystal layer 19 has a larger band gap and a lower refractive index than the buried active layer 13, a buried refractive index waveguide structure is formed and stable transverse mode oscillation is performed. Therefore, similar to the first embodiment, it is possible to reduce the oscillation threshold and improve the temperature characteristics. Further, in this embodiment, all the semiconductor layers can be formed by one crystal growth, so there is an advantage that the manufacturing process can be simplified.

FIG. 6 is a cross-sectional view showing the manufacturing process of a semiconductor laser according to a fifth embodiment of the present invention. Furthermore, Figure 1
The same parts are given the same reference numerals, and detailed explanation thereof will be omitted.

In this embodiment, as shown in FIG. 6(a), after forming up to the current conduction facilitating layer 18 as in the fourth embodiment, an n-GaAs current confinement layer 61 with a thickness of 1.5 μm is formed. Then, a 0.2 μm thick SiO 2 film 16 having stripe-shaped openings with a width of 2 μm is formed thereon. Thereafter, in the same manner as in the fourth embodiment, the MQW active layer 13 directly under the SiO2 film 16 is mixed crystal, and the MQW structure is buried with the mixed crystal layer 19.

Next, as shown in FIG. 6(b), SiO
2 After removing the film 16, a p-GaAs contact layer 20 with a thickness of 3 μm is grown so as to bury the current confinement layer 61, and finally, AuZn/Au is grown as the p-side electrode 21 and AuGe/Au is grown as the n-side electrode 22. Au was formed. The buried MQW laser obtained here oscillates on the same operating principle as that obtained in the fourth example, and electrodes can be easily formed.

FIG. 7 is a diagram for explaining the manufacturing process of a semiconductor laser according to a sixth embodiment of the present invention.
is a perspective view, (b) and (d) are sectional views taken along arrow AA' in (a), and (c) and (e) are sectional views taken along arrow BB' in (a). In this example, the method of the fifth example is used for both forming the window structure and the buried structure.

In the same manner as in the fifth embodiment, as shown in FIG. 7(a), after forming up to the n-GaAs current confinement layer 61 with a thickness of 1.5 μm, for example, a stripe width of 2 μm and a stripe length of A SiO2 mask 16 with a thickness of 0.2 μm and having a rectangular opening with a diameter of 400 μm is formed. Next, as shown in FIGS. 7(b) and 7(c), the SiO2 film 1
The MQW active layer 13 directly under the active layer 6 is mixed crystal, and a window structure is formed at the same time as the MQW buried structure.

Next, as shown in FIGS. 7(d) and (e),
By etching the current confinement layer 61 using the SiO2 film 16 as a mask until the current conduction facilitation layer 18 is exposed,
Forms a current confinement structure. Subsequently, after removing the SiO2 film 16, a p-GaAs contact layer 20 with a thickness of 3 μm is grown, and AuZn/Au is used as the p-side electrode 21.
AuGe/Au was formed as the n-side electrode 22.

The window-structured buried MQW laser obtained here oscillates on the same operating principle as in the fifth embodiment. Furthermore, since the mixed crystal layer 19 is formed by heat treatment rather than by impurity diffusion, a non-injection type window structure can be realized, and end face breakage is less likely to occur and high output operation is possible.

It should be noted that the present invention is not limited to the above-mentioned embodiments. For example, in the first to fifth embodiments as well, it is possible to simultaneously form the window structure and the embedded structure in the same manner as in the sixth embodiment. At this time as well, the 6th
It is possible to obtain an embedded MQW laser with a non-injection type window structure that oscillates on the same operating principle as in the embodiment.

Furthermore, the active layer is not limited to the MQW structure, and if the active layer is as thin as 0.03 μm or less, it can also be applied to a double heterostructure laser in which the active layer is a bulk. Furthermore, although the embodiments have been mainly described with reference to an InGaAlP semiconductor laser, other materials may also be used. Furthermore, the dielectric film is not limited to SiO2.
It is also possible to use a Si3 N4 film. others,
Various modifications can be made without departing from the spirit of the invention.

[0045]

As described in detail above, according to the present invention, instead of using a mixed crystal layer mixed by impurity diffusion in the buried layer of a buried semiconductor laser and the window part of a window structure semiconductor laser, By mixing the active layer through heat treatment to form a mixed crystal layer, it is possible to form a mixed crystal layer with a low carrier concentration, which contributes to lowering the oscillation threshold and increasing output. It becomes possible to manufacture a semiconductor laser that obtains the desired results.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the manufacturing process of a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view showing the main configuration of FIG. 1;

FIG. 3 is a sectional view showing a schematic structure of a second embodiment of the present invention.

FIG. 4 is a sectional view showing a schematic structure of a third embodiment of the present invention.

FIG. 5 is a process sectional view for explaining a fourth embodiment of the present invention.

FIG. 6 is a process sectional view for explaining a fifth embodiment of the present invention.

FIG. 7 is a perspective view and a sectional view for explaining a sixth embodiment of the present invention.

FIG. 8 is a process cross-sectional view for explaining a conventional semiconductor laser manufacturing method.

[Explanation of symbols]

10... n-GaAs substrate (compound semiconductor substrate), 11...
n-In0.5 (Ga0.3 Al0.7)0.5
P cladding layer, 12...In0.5 (Ga0.5 Al0.5)0.
5 P light guide layer, 13...Multiple quantum well (MQW) active layer, 13a...In0
．． 5 Ga0.5 P well layer, 13b...In0.5 (
Ga0.5 Al0.5 )0.5 P barrier layer 14...
In0.5 (Ga0.5 Al0.5)0.5 P
Light guide layer, 15...p-In0.5 (Ga0.3 Al0.7)
0.5 P cladding layer, 16...SiO2 film (dielectric film), 17...p-In0.5 (Ga0.3 Al0.7)
0.5 P cladding layer, 18...p-In0.5 Ga0.5 P current conduction facilitation layer,
19... Mixed crystal layer, 20... p-GaAs contact layer, 21, 41... p-side electrode, 22... n-side electrode, 33... In0.5 Ga0.5 P active layer, 61... n-
GaAs current confinement layer.

Claims (1)

[Claims] 1. A step of forming a double heterostructure on a compound semiconductor substrate with an active layer sandwiched between cladding layers; and a step of forming a dielectric film having a partial opening on the double heterostructure. A method of manufacturing a semiconductor laser, comprising the step of: then subjecting the substrate to heat treatment to make a portion of the active layer directly under the dielectric film a mixed crystal.