JPH0992924A - Semiconductor laser - Google Patents

Abstract

(57) Abstract: A stable semiconductor laser whose oscillation wavelength is fixed even if the operating temperature changes is provided. At least a part of an optical resonator constituting a semiconductor laser has a dielectric having a negative refractive index change with respect to temperature (for example, lithium fluoride; refractive index = −1.6 ×).
10 ⁻⁵ (K ⁻¹ )) 13 and a dielectric (for example, calcium fluoride; refractive index = −1.0 × 10 ⁻⁵ (K ⁻¹ )) 14.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to wavelength stabilization of a semiconductor laser, and the semiconductor laser of the present invention can provide a stable light source whose oscillation wavelength is fixed even when the operating temperature changes. It will be possible.

[0002]

2. Description of the Related Art A semiconductor laser is practically used as a device constituting an optical communication system or an optical information processing system because it is small in size, operates with low power consumption and is capable of high speed modulation.

FIG. 8 shows a schematic sectional structure of a conventional semiconductor laser. In FIG. 8, 1 is a p-side electrode, 2 is a contact layer,
3 is a p-side InP clad layer, 4 is a light confinement layer, 5 is a light emitting layer, 6 is a light confinement layer, 7 is an InP substrate, 8 is an n-side electrode, 9 is a silicon oxide film, 10 is a titanium oxide film, and 11 is InP layer, 12 shows an InGaAsP layer. FIG.
(A) shows the case where the cleaved surface of the crystal is used as a reflecting mirror, and (b) shows the case where a multilayer film made of a dielectric film such as a silicon oxide film 9 and a titanium oxide film 10 formed on the cleaved surface is a reflecting mirror. (C) is an InP layer 11 or InG which is at least two kinds of soot with different refractive indexes.
The case where the periodic structure of the aAsP layer 12 is used as a reflecting mirror is shown.

[0004]

However, in these semiconductor lasers, the refractive index of the constituent soot increases as the temperature rises, the length of the resonator increases due to thermal expansion, and the active layer Since the wavelength of the light emitted by the light also moves to the long wavelength side, the oscillation wavelength moves to the long wavelength side. Therefore, when constructing a transmission system such as wavelength division multiplexing, a temperature adjustment function using a Peltier element or the like is added to keep the oscillation wavelength constant, or a semiconductor laser with a wavelength variable mechanism such as a multi-electrode DBR laser is used. It was necessary to keep the oscillation wavelength constant by adjusting the current injection. As a result, the power consumption increases, the size of the device itself increases, and the cost increases.

The present invention has been proposed in order to improve the above-mentioned drawbacks, and an object thereof is to provide a semiconductor laser capable of keeping the oscillation wavelength constant without being influenced by the operating temperature.

[0006]

In order to solve the above problems, according to the structure of the semiconductor laser of the present invention, at least a part of the optical resonator forming the semiconductor laser has a negative refractive index change with temperature. It is characterized by being composed of a material.

Further, in the above structure, there are at least two gain regions located in the optical resonator.

Further, in the above structure, the composition of the active layer forming the gain region located in the optical resonator is made of a semiconductor that continuously or discretely changes in the film direction. And

Here, in the semiconductor laser of the present invention, since a medium having a negative refractive index change with respect to temperature is used in at least a part of the portion constituting the semiconductor laser as described above, other than these media. This cancels out the phase change of the light generated in the area, and the oscillation wavelength is kept constant.

Further, by changing the composition of the light emitting region continuously or discretely, a constant gain value can be obtained over a wide wavelength range, so that the absorption edge of the active layer changes due to temperature change. Also, the gain value can be kept constant, and oscillation can be continued at a constant wavelength even if a large temperature change occurs.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described below in detail based on the embodiments shown in the drawings.

(First Embodiment) FIG. 1 is a sectional view of a basic structure showing a first embodiment of a semiconductor laser according to the present invention. In the figure, reference numeral 7 is a semiconductor substrate of single crystal InP, 4 and 6 are optical confinement layers made of single crystal InGaAsP, and 5 has an emission wavelength peak of 1.55 μm.
Is a light emitting layer, and 2 is a single crystal In for taking an electrode.
Contact layers 1 and 8 made of GaAsP are metal for electrodes. Reference numerals 13 and 14 are dielectric films having a negative refractive index change with temperature, and their thicknesses are λ / 4 /, where n is the refractive index of the medium and λ is the laser oscillation wavelength.
n.

Next, each part of the element shown in FIG. 1 will be described in detail. After n-type InP is grown to a thickness of about 100 nm on the n-type InP substrate 7, the forbidden band width is 1.3 μm.
InGaAsP layer 6 containing no impurities corresponding to m
(Thickness 0.1 μm) is laminated. Then, 5 pairs of InGaAs (thickness 8 nm) lattice-matched with the InP substrate and InGaAsP (thickness 10 nm) whose forbidden band width corresponds to 1.3 μm in wavelength are laminated to form a light emitting layer 5 composed of multiple quantum wells. . On the light emitting layer 5, the forbidden band has a wavelength of 1.
InGaAsP layer 4 (thickness 0.1 μm) and p-type InP layer 3 (thickness 1.5 μm) corresponding to 3 μm and containing no impurities.
m) are sequentially laminated. Further thereon, a p-type contact layer 2 (thickness 0.3 μm) that lattice-matches the InP substrate is laminated.

Next, a stripe pattern having a length of 100 μm is formed by using a photoresist, and is vertically etched to the bottom of the light confinement layer 6 by dry etching. After removing the photoresist, lithium fluoride, which is a dielectric material 13 having a low refractive index, is deposited by an electron beam evaporation method. After vapor deposition, the surface is flattened, and then a resist pattern is formed by an electron beam exposure method.

This pattern covers the entire upper surface of the semiconductor, and extends to a position 280 nm away from the semiconductor end face formed by dry etching, and 272 from that position.
The part where the nm pattern does not exist and the part where the 280 nm pattern exists repeatedly exist. Using this pattern, dry etching is performed to a depth reaching below the light confinement layer 6. Next, calcium fluoride, which is a high-refractive-index dielectric material 14, is vapor-deposited in the periodic grooves formed by the dry etching by an electron beam vapor deposition method.

After the surface is flattened again and the semiconductor surface is exposed, the InP substrate 7 is polished to a thickness of about 100 μm. Ni, Zn and Au are deposited on the contact layer 2 by 5
Ni, Ge, and Au are vapor-deposited on the entire surface on the InP substrate side with a width of 0 μm by electron beam vapor deposition. After that, heat treatment is performed at 420 ° C. for 20 seconds in a hydrogen atmosphere.

After that, the total length of the semiconductor portion is 20.
Cut out as a 0 μm element. The refractive indexes of lithium fluoride and calcium fluoride of the dielectrics 13 and 14 become smaller as the temperature rises. The respective values are −1.6 × 10 in the refractive index of lithium fluoride of the low refractive index dielectric material 13.
^-5 (K ^-1 ) and the refractive index of calcium fluoride of the one high-refractive-index dielectric material 14 is -1.0 × 10 ^-5 (K ^-1 ). As the operating temperature rises, the refractive index of the semiconductor part increases to 2.7 × 10 ^-5 (K ^-1 ), but 1.55 μm
The portion where the phase of the oscillated light is changed can be canceled by the portion of lithium fluoride 13 and calcium fluoride 14,
It can continue to oscillate at a constant wavelength.

In this case, since the buried structure is not adopted, the oscillation threshold current at 20 ° C. was 85 mA.
The change in the oscillation wavelength when the operating temperature was changed from 0 ° C. to 80 ° C. was 5Å, which was less than one tenth of the conventional change. The amount of change in the threshold current at that time was 90 mA.

(Second Embodiment) FIG. 2 shows a second embodiment of the present invention. In the present embodiment, the light emitting layer 5 is formed by linearly changing the flow rate of the supply gas from a composition corresponding to a forbidden band width of 1.4 μm in wavelength to a composition corresponding to 1.6 μm.

In the embodiment shown in FIG. 2, unlike the first embodiment, lithium fluoride, which is the dielectric 13 of the medium exhibiting a negative refractive index change with temperature, contacts the light emitting layer portion. I try not to do it. In this case, the gap d formed between the medium and the side surface of the light emitting portion is 388 nm obtained by dividing the oscillation wavelength by 4 times the refractive index of air.

In this device structure, when the resist pattern is formed by the electron beam exposure method in the above process, the pattern is formed from the entire upper surface of the semiconductor and the semiconductor end surface.
It is configured such that the 88 nm portion, and the portion outside the 88 nm portion, the portion without the 272 nm pattern and the portion with the 280 nm pattern are repeated. Therefore, in this embodiment, after the calcium fluoride 14 is deposited, only the lithium fluoride 13 in contact with the light emitting layer is removed by dry etching. After that, planarization and substrate polishing are performed to form electrodes.

Also in this case, the total length is 2 as in the first embodiment.
When cut out as a device of 00 μm, the oscillation threshold value at 20 ° C. was 110 mA, which was higher than that in the first embodiment, but the change in the oscillation wavelength when the operating temperature was changed from 20 ° C. to 80 ° C. It was 4Å, and the amount of change in the threshold current was 20 mA, which was about one fifth of that in the first embodiment.

(Third Embodiment) FIG. 3 shows a third embodiment of the present invention. This structure has a structure as a so-called composite resonator laser added in order to emphasize the single mode oscillation of the semiconductor laser.

This device is produced by forming a pattern with a photoresist after forming the structure shown in FIG. 2 and vertically etching a semiconductor by dry etching. In this case, since the standing wave that can be commonly present in each of the divided portions can oscillate most stably, the temperature range of single-mode oscillation is wide, and the oscillation occurs in single mode from 20 ° C to 105 ° C.

Although the semiconductor end face is a gap formed by air in the above embodiment, the medium having the smaller refractive index may be in contact with the semiconductor side face as shown in FIG.

In the above-mentioned first, second and third embodiments, the periodic structure of the dielectric film is formed by using the electron beam evaporation method and the dry etching, but the present invention is not limited to this, and for example, the deposition of the dielectric film. Alternatively, the sputtering method or the CVD method may be used, and the periodic structure may be formed by an ion implantation method, an ion exchange method, a thermal diffusion method, or the like.

(Fourth Embodiment) FIG. 4A,
(B) shows a fourth embodiment of the present invention. First,
The InP substrate 7 of the semiconductor wafer is polished, and the electrode metals 1 and 8 are deposited. And 20 at 420 ℃ in hydrogen atmosphere
Heat treatment is performed for a second. After that, a gap of about 150 μm is formed under the light confinement layer 6 by dry etching.
Then, sodium chloride, which is the dielectric material 15a, is deposited up to the optical confinement layer 4 by vacuum evaporation to fill the gap.

Then, as shown in FIG. 4B, sodium chloride is etched by dry etching to a width equal to that of the electrode 1. Sodium chloride 15
Sodium fluoride, which is a dielectric material 15b having a smaller refractive index than a, is deposited to form a waveguide structure between the divided active layers. Next, etching is performed until the electrode 1 is exposed. Then, a laser element having a total length of about 400 μm is cut out.

In this case, the oscillation threshold current at 20 ° C. is 10
The value was 0 mA, and the change in the oscillation wavelength was 4Å when the operating temperature was changed from 20 ° C to 50 ° C. The amount of change in the threshold current at that time was 45 mA.

In the fourth embodiment, the dielectric is in contact with the side surface of the semiconductor, but a gap may be formed by air by dry etching or the like.

(Fifth Embodiment) FIG. 5 shows a fifth embodiment of the present invention. In this embodiment, after the n-type InP substrate is grown to a thickness of about 100 nm on the n-type InP substrate 7, the InGaAsP layer 6 (thickness: 0. 1 μm) is laminated. Next, InGa that is lattice-matched with the InP substrate
As (thickness 8 nm) and forbidden band width is 1.3 μm
5 pairs of InGaAsP (thickness: 10 nm) corresponding to the above are laminated to form a light emitting layer 5 composed of multiple quantum wells. Light emitting layer 5
An InGaAsP layer 4 (thickness of 0.1 μm) having no forbidden band width corresponding to 1.3 μm in wavelength is added on the top.
And p-type InP (thickness 0.3 μm) are stacked. A 40 .mu.m.times.200 .mu.m silicon oxide pattern is formed on top of this, and etching is performed below the light confinement layer 6 using this film as a mask to form island-shaped light emitting regions.

Next, using the above mask as it is, InGa
Buried growth of the AsP layer 12 is performed. After removing the mask, a resist pattern for a diffraction grating is formed outside the island-shaped light emitting region in the major axis direction of the island-shaped light emitting region by the electron beam exposure method.
Etching is performed using this resist pattern as a mask to form a semiconductor diffraction grating. Then, the InP layer 11 is grown on the entire surface to flatten the surface.

Next, a stripe pattern of a silicon oxide film having a width of 1.5 μm that passes through the diffraction grating and the upper part of the island-shaped light emitting layer is formed. Then, using this silicon oxide as a mask, etching is performed to the bottom of the light confinement layer 6, and the surroundings are continuously filled with p-type InP and n-type InP. Next, the silicon oxide film is removed, and the p-type InP layer 3 and the p-type contact layer 2 are formed on the entire surface.
To grow.

Next, a 100 μm window is opened with a photoresist on the upper surface of the portion including the light emitting layer 5. Then, using this resist as a mask, dry etching is performed to divide the light emitting region. After that, the resist is removed, and calcium fluoride, which is a dielectric 15a having a negative refractive index change, is vapor-deposited to the height of the optical confinement layer 4. Next, in the same manner as in Embodiment 4 described above, on the calcium fluoride 15a,
A resist stripe pattern having a width similar to that of the buried active layer is formed on the same straight line as the stripe-shaped active layer. Calcium fluoride is etched by dry etching using this resist as a mask. After removing the resist, the dielectric 1 having a smaller refractive index than calcium chloride 15a as a whole
Sodium fluoride which is 5b is vapor-deposited to form a waveguide structure.

After that, the surface is flattened and the semiconductor is exposed. Then, the substrate 7 is polished and the electrode metals 1 and 8 are formed on the surface,
It is vapor-deposited on the back surface and heat-treated. In this case, the oscillation threshold current at 20 ° C. was 15 mA, and the change in oscillation wavelength when the operating temperature was changed from 20 ° C. to 85 ° C. was 5Å. The amount of change in the threshold current at that time was 35 mA.

(Sixth Embodiment) FIG. 6 shows a surface emitting laser according to a sixth embodiment of the present invention. In this embodiment, on the n-type InP substrate 7,
After n-type InP is grown to about 100 nm, the n-type InGaAsP layer 19 has a forbidden band width of 1.4 μm.
(Thickness 0.11 μm) and n-type InP18 (thickness 1.2 μm)
m), and an InGaAsP layer 4 (thickness: 0.1 μm) corresponding to a wavelength band gap of 1.3 μm corresponding to a wavelength.
m) are laminated. Next, I which is lattice-matched with the InP substrate
nGaAs (thickness 8 nm) and forbidden band width are 1.
InGaAsP (thickness 10 nm) corresponding to 3 μm is 1
2.5 pairs of layers are stacked to form a light emitting layer 5 composed of multiple quantum wells. On the light emitting layer, an InGaAsP layer 6 (thickness: 0.1 μm) and a p-type InP (thickness: 0.5 μm) having no band gap corresponding to 1.3 μm in wavelength are stacked.

Next, a circular silicon oxide film having a diameter of 15 μm is formed on the surface and is etched by about 1.5 μm using this as a mask. After that, the p-type InP layer 17 and the n-type InP layer 16 are continuously grown to form a buried structure. Then, the silicon oxide film is removed, and p-type InP3 and a p-type InGaAsP layer (thickness 0.35 μm) 2 having a forbidden band width of 1.4 μm in wavelength are laminated on the entire surface.

Next, the electrode metal 1 is deposited on the semiconductor surface so as not to cover the light emitting region. Next, the substrate 7 is polished to a thickness of about 100 μm, and the electrode metal 8 is deposited. Four
After heat treatment at 20 ℃ for 20 seconds, 200μm × 300μ
forming a window pattern for extracting light of m on the electrode metal 8;
InGaAsP layer 1 by etching electrode metal 8 and substrate 7
Put 9 on the surface. Then, after depositing about 1 μm of calcium fluoride, which is the dielectric 15 having a negative refractive index change, on the semiconductor surface on the electrode 1 side, the silicon oxide film 9 (0.26 μm) is deposited.
m) and titanium oxide film 10 (0.18 μm) are vapor-deposited in 12 pairs. The diameter is 30 μm so as to cover the light emitting region.
The resist pattern of 1 is formed on the surface, and dry etching is performed until the electrode 1 appears on the surface.

Further, the silicon oxide film 9 (0.
26 μm) and titanium oxide film 10 (0.18 μm) 12
Pair vapor deposition. The structure of FIG. 6 is completed by leaving the resist pattern only on the etched portion of the substrate 7 and exposing the electrode 8 by dry etching.

In this case, the oscillation threshold current in the pulse operation at 20 ° C. was 35 mA, and the change in the oscillation wavelength when the operating temperature was changed from 20 ° C. to 50 ° C. was 6Å. The amount of change in the threshold current at that time was 55 mA.

(Seventh Embodiment) FIG. 7 shows a surface emitting laser according to a seventh embodiment of the present invention. In this embodiment, an n-type InP substrate 7 is provided with an n-type InP substrate.
After growing about 100 nm, the forbidden band width is changed to 1.
The n-type InGaAsP layer 19 (thickness of 0.
11 μm), an n-type InP layer 18 (thickness: 1.2 μm), and an InGaAsP layer 6 (thickness: 0.1 μm) having no forbidden band width of 1.3 μm in terms of wavelength. Next, InGa that is lattice-matched with the InP substrate
As (thickness 8 nm) and forbidden band width is 1.3 μm
InGaAsP (thickness 10 nm) corresponding to 12.5
The light emitting layer 5 composed of multiple quantum wells is formed by stacking pairs. On the light emitting layer, an InGaAsP layer 4 (thickness: 0.1 μm) corresponding to a forbidden band width of 1.3 μm in wavelength is added.
m) and a p-type InP layer (thickness 0.5 μm) are laminated.

Next, a circular silicon oxide film having a diameter of 15 μm is formed on the surface and is etched by about 1.5 μm using this as a mask. After that, the p-type InP layer 17 and the n-type InP layer 16 are continuously grown to form a buried structure. Then, the silicon oxide film is removed, and the p-type InP layer 3 and the p-type InGaAsP layer 2 having a forbidden band width of 1.4 μm in wavelength are formed on the entire surface.
(Thickness 0.35 μm) is laminated. Next, the electrode metal 1 is deposited on the semiconductor surface so as not to cover the light emitting region.

Next, the substrate 7 is polished to a thickness of about 100 μm, and the electrode metal 8 is deposited. After heat treatment at 420 ° C. for 20 seconds, a light extraction window pattern of 200 μm × 300 μm is formed on the electrode metal 8, and the electrode metal 8 and the substrate 7 are formed.
Is etched. After that, 0.28 μm of lithium fluoride, which is a dielectric 13 having a negative refractive index change on the semiconductor surface on the electrode 1 side, and 0.22 μm of cesium iodide, which is a dielectric 14 having a higher refractive index than the dielectric 13. 20 pairs are vapor-deposited.

The diameter is 30 μm so as to cover the light emitting region.
The resist pattern of 1 is formed on the surface, and dry etching is performed until the electrode 1 appears on the surface. Similarly, 20 pairs of lithium fluoride 0.28 μm and cesium iodide 0.22 μm are vapor-deposited on the back surface side. The structure of FIG. 7 is completed by leaving the resist pattern only on the etched portion of the substrate 7 and exposing the electrode 8 by dry etching.

In this case, the oscillation threshold current in the pulse operation at 20 ° C. was 28 mA, and the change in the oscillation wavelength when the operating temperature was changed from 20 ° C. to 60 ° C. was 2Å. The amount of change in the threshold current at that time was 35 mA.

In the above-described sixth and seventh embodiments, the dielectric multilayer film is used for both the front surface and the back surface, but a semiconductor multilayer film may be used. Further, it does not have to be an embedded structure.

In the second embodiment, the composition of the light emitting layer is continuously changed, but it may be changed discretely.
Further, a multiple quantum well structure, a strained quantum well structure, or a bulk structure may be used. Embodiment Modes 1, 3, 4,
Although 5, 6 and 7 are light emitting layers having a multiple quantum well structure, the structure of the light emitting layers may be a park, and Embodiment 2
As described above, the composition may be changed continuously or discretely.

The above-described first, second, third, fourth, and fifth embodiments may have a buried structure, or a dielectric may have a waveguide structure.

In the above embodiment, lithium fluoride,
Although calcium fluoride and sodium chloride are used, the present invention is not limited to this, and in addition to the above, for example, potassium chloride, cesium bromide, potassium bromide, sodium fluoride,
Potassium iodide, cesium iodide, KRS-5 (thalli
Similar effects can be expected with um-bromide-iodide), ADP (ammonium dihydrogen phosphate), KDP (potassium dihydrogen phosphate) and the like. The amount of change in the refractive index with respect to the temperature of these media is summarized in "Table 1" below.

[0050]

[Table 1]

The larger the amount of change in the refractive index with respect to temperature, the more effectively the phase can be adjusted. The same effect can be expected by implanting a rare earth element such as praseodymium, neodymium or erbium into the silicon oxide film.

The smaller the ratio of the length of the light emitting region occupied in the laser resonator, the more stable the oscillation at a constant wavelength even with a large temperature change. In addition, the first and second embodiments
In 3 and 8, the smaller the difference in refractive index between the two types of dielectric films used, the stronger the phase adjustment effect.

[0053]

As described above, the semiconductor laser of the present invention uses a medium exhibiting a negative refractive index change with temperature or a medium exhibiting a negative linear thermal expansion coefficient with respect to temperature. This cancels the phase change of the light generated in the part other than the medium, and the oscillation wavelength is kept constant.

By changing the composition of the light emitting region continuously or discretely, a constant gain value can be obtained over a wide oscillation range, so that the absorption edge of the active layer changes due to temperature change. Also, the gain value can be kept constant, and oscillation can be continued at a constant wavelength even if a large temperature change occurs.

[Brief description of drawings]

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

FIG. 2 shows a sectional view of a second embodiment of a semiconductor laser according to the present invention.

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

4A and 4B are cross-sectional views of a fourth embodiment of a semiconductor laser according to the present invention, where FIG. 4A shows a cross section in the major axis direction and FIG. 4B shows a cross section in the minor axis direction.

FIG. 5 is a sectional view showing a fifth embodiment of a semiconductor laser according to the present invention.

FIG. 6 is a sectional view of a semiconductor laser according to a sixth embodiment of the present invention.

FIG. 7 is a sectional view of a semiconductor laser according to a seventh embodiment of the present invention.

FIG. 8 is a cross-sectional view of a conventional semiconductor laser.

[Explanation of symbols]

 1 p-side electrode 2 contact layer 3 p-side InP clad layer 4 optical confinement layer 5 light-emitting layer 6 optical confinement layer 7 InP substrate 8 n-side electrode 9 silicon oxide film 10 titanium oxide film 11 InP layer 12 InGaAsP layer 13 low refractive index dielectric Body 14 High refractive index dielectric material 15, 15a, 15b Dielectric material 16 n-type InP 17 p-type InP 18 n-type InP clad layer 19 InGaAsP layer

Claims (3)

[Claims] 1. A semiconductor laser, wherein at least a part of an optical resonator constituting the semiconductor laser is made of a material having a negative refractive index change with temperature. 2. The semiconductor laser according to claim 1, wherein there are at least two gain regions located in the optical resonator. 3. The semiconductor laser according to claim 2 or 3, wherein the composition of the active layer forming the gain region located in the optical resonator changes continuously or discretely in the film direction. A semiconductor laser characterized in that