JPH0691295B2 - Semiconductor laser and manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor laser and a method for manufacturing the same.

[Prior Art] A semiconductor laser formed on a semi-insulating substrate is expected to have excellent high-frequency characteristics because the parasitic capacitance generated by the electrodes is extremely small. Conventional examples of such semiconductor lasers (Proceedings of the 2nd Semiconductor Laser Symposium, p.10, 1985)
Issued on February 1, 2012) is shown in Fig. 3. In FIG. 3, 21
Is an n-type cladding layer, 16 is an active region, 22 is a p-type cladding layer, and the current is a semi-insulating In located on both sides of the active region 16.
It is narrowed by P. Therefore, the electrons are injected into the active region 16 through the n-type electrode 14 and the n-type clad layer 21, while the holes are p-type electrode 17, the Zn diffusion layer 23, the p ^{+ type} conductive layer 25, and the p-type clad layer 22.
Is injected through. In addition, the parasitic capacitance of the electrode is semi-insulating In
Since the P substrate 10 is used, it is extremely small, and this semiconductor laser exhibits a high frequency response characteristic. Incidentally, reference numeral 15 in the drawing is an insulating film.

[Problems to be solved by the invention]

However, the conventional semiconductor laser formed on the semi-insulating substrate has a drawback that it is difficult to unify the oscillation axis modes that are essential for high-speed transmission. This is because, in this semiconductor laser, since the double hetero structure including the active layer is grown in the groove, it is impossible to provide the diffraction grating on the plane parallel to the substrate.

As one measure against this problem, there is a method (Japanese Patent Laid-Open No. 59-4190) in which a periodic concave and convex is formed on the side surface of the groove to form a diffraction grating. However, since the light intensity distribution is small near the side surface of the groove corresponding to the edge of the waveguide, the coupling coefficient of the diffraction grating is not large enough to unify the axial modes in this method, and the multimode There was a drawback that it was easy to change.

Further, when the active layer is formed in the groove provided in the substrate, the growth characteristics peculiar to the liquid phase growth are used, and therefore the liquid phase growth is unavoidable. As a result, since the controllability of liquid phase growth is poor, the reproducibility and uniformity of the thickness and width of the active layer are poor, making it difficult to obtain devices with a high yield.

[Object of the Invention]

An object of the present invention is to provide a semiconductor laser having a low parasitic capacitance, capable of high-speed modulation, oscillating in a single axis mode, and having a structure excellent in controllability, and thus a high manufacturing yield can be expected, and a manufacturing method thereof. To provide.

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention has an active layer laminated in a direction perpendicular to a substrate surface and an active region formed of an optical guide layer having a forbidden band width larger than the forbidden band width of the active layer, to the left and right, A structure in which first and second clad portions are arranged is provided on a semi-insulating semiconductor substrate, and the first clad portion has two half bands having a forbidden band width larger than the forbidden band width of the light guide layer. A semiconductor having a laminated structure sandwiched vertically by insulating semiconductor layers, the second cladding portion having a conductivity type opposite to that of the semiconductor layer, and having a forbidden band width larger than the forbidden band width of the active layer. The optical waveguide layer and the first cladding portion have periodic irregularities at the interface, and electrodes are provided on the first cladding portion and the second cladding portion, respectively.

Further, in order to achieve the above object, the present invention sandwiches a conductive layer of the first conductivity type on a semi-insulating semiconductor substrate with a semi-insulating semiconductor layer having a forbidden band width larger than the forbidden band width of the conductive layer. A step of forming a laminated structure, a step of forming a stepped shape having a periodic unevenness on a side surface and having a depth reaching the semi-insulating semiconductor substrate in the laminated structure, and a step of forming the conductive layer on the side surface. An optical guide layer having a forbidden band width smaller than the forbidden band width and an active layer having a forbidden band width smaller than the forbidden band width of the optical guide layer are formed, and then a clad of the second conductivity type is formed so as to fill the step shape. And a step of forming a layer.

[Action]

The semiconductor laser according to the present invention has a structure in which the optical guide layer and the active layer are laterally laminated on the diffraction grating formed on the side wall of the first cladding portion. Therefore, the light intensity distribution in the vicinity of the diffraction grating becomes sufficiently large, and a coupling coefficient capable of uniaxial mode oscillation can be obtained. Further, of the two types of carriers injected into the active region, the carrier passing through the first clad portion is narrowed down to the conductivity type semiconductor by the two semi-insulating semiconductor layers forming the first clad portion.

Therefore, the injection current flows only in the active region in the active layer, the leakage current can be remarkably reduced as compared with the conventional one, and oscillation can be performed with high efficiency. Further, since there is no structure in which SiO ₂ which is a conventional parasitic capacitance generation site is sandwiched between the electrode and the highly doped semiconductor, the residual capacitance can be remarkably reduced as compared with the conventional one.

The semiconductor laser is realized by the manufacturing method according to the present invention. Since both the thickness and the width of the active region are determined by the accuracy of epitaxial growth, the present method using the epitaxial growth method having high controllability such as vapor phase growth can be controlled with high accuracy. Moreover, if electron beam exposure and reactive ion etching are used to form the diffraction grating, the depth, shape, and period of the diffraction grating can be controlled with high accuracy and good reproducibility.

As described above, the manufacturing method of the present invention is a method with high reproducibility and controllability using vapor phase growth, electron beam exposure, and reactive ion etching. According to this method, high yield modulation with high yield is achieved. A possible semiconductor laser can be obtained.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a view for explaining an embodiment of a semiconductor laser of the present invention, and is a view showing cross-sectional shapes on a plane parallel to a substrate and a plane perpendicular to the substrate. A part of the InGaAsP active layer 19 having a forbidden band width of 0.8 eV and a part of the InGaAsP optical guide layer 20 having a forbidden band width of 0.95 eV are optical waveguide regions (active regions) 24.
Light is emitted at a wavelength determined by the propagation constant of 24, the period of the diffraction grating formed in the optical guide layer 20, and the gain spectrum of the active layer 19.

The first cladding part is a semi-insulating buffer layer 1 of InP composition.
1, n ^{+ type} conductive layer composed of InGaAsP layer with a band gap of 1.2 eV 1
2 and a semi-insulating cap layer 13 of InP composition, and the second cladding portion is composed of a p-type conductive layer 18 of InP composition. The optical waveguide region 24 is sandwiched between the first cladding portion and the second cladding portion from the left and right to form a double heterojunction. Further, by making the composition of the n ⁺ -type conductive layer 12 an InGaAsP layer having a refractive index larger than that of InP, a refractive index guiding desirable for a semiconductor laser for optical communication is realized.

The layer thicknesses of the active layer 19 and the light guide layer 20 forming the optical waveguide region 24 are the same as or smaller than the guide wavelength. Therefore, a sufficiently large guided light also exists in the light guide layer 20 having a diffraction grating, and therefore, the coupling coefficient of the diffraction grating has the same value as the coupling coefficient in a normal DFB laser or the like. as a result,
Single axis mode oscillation was obtained.

Of the carriers injected into the active region 24, electrons are injected from the n-type electrode 14 through the n ⁺ -type conductive layer 12, but due to the high resistance of the semi-insulating cap layer 13 and the semi-insulating buffer layer 11, the active region. Electron leakage to the InGaAsP active layer 19 other than the above can be ignored. On the other hand, holes are transmitted from the p-type electrode 17 through the p-type conductive layer 18.
Although injected into the InGaAsP active layer 19, recombination with electrons mainly occurs in the active region. Therefore, the injection current is
Therefore, high-efficiency oscillation is possible.
Further, since the electrodes 14 and 17 are separated by the SiO ₂ layer 15, the leakage current between the electrodes can be ignored.

The parasitic capacitance is generated between the n-type electrode 14 and the p-type electrode 17 separated by the SiO ₂ layer 15. In addition, it is generated between the heat sink and the n-type electrode 14 or the p-type electrode 17 which are placed to face each other with the semi-insulating semiconductor substrate 10 interposed therebetween. Here, the electrode spacing, the resonator length, and the substrate thickness are 5 μm and 300, respectively.
When the thickness was 100 μm and 100 μm, the total parasitic capacitance was an extremely low value of 0.05 pF.

As a result, even if a relatively large residual inductance of about 3 nH occurs due to mounting such as wire bonding, the cutoff frequency is 13 GHz or higher. Thereby, 10Gb / s
High-speed modulation is now possible.

Next, an example of the manufacturing method of the present invention will be described. In this embodiment, a case of manufacturing the semiconductor laser having the structure shown in FIG. 1 will be described. FIG. 2 shows cross-sectional views at each stage of the manufacturing process.

First, as the first configuration, the iron-doped semi-insulating InP buffer layer 11 is formed on the iron-doped semi-insulating InP substrate 10 by vapor phase epitaxy.
(Thickness 2.5 μm), sulfur-doped n ^{+ -type} InGaAsP conductive layer 12 (n
= 1 × 10 ¹⁸ cm ^-3 , bandgap 1.2eV, thickness 1.5μ
m), iron-doped semi-insulating InP cap layer 13 (thickness 0.8μ
m) was formed by vapor phase epitaxy (see FIG. 2 (a)).

Next, as a second step, a stepped shape having periodic unevenness on the side wall is formed on this laminated structure by using an electron beam exposure method and reactive ion etching. For this purpose, first, a SiO ₂ mask layer 27 and a Ti mask layer 26 are sequentially formed on this laminated structure, and then an electron beam resist is applied and an electron beam exposure method is used to produce an electron beam as shown in FIG. 2 (b). A pattern of line resist 30 was formed. The depth and period of the unevenness of this pattern were 100 nm and 480 nm, respectively. continue,
After etching the Ti mask layer 26 and the SiO ₂ mask layer 27 using hydrofluoric acid using the electron beam resist 30 as a mask,
By using the reactive ion etching method using the Ti mask layer 26 as a mask, a stepped shape having a diffraction grating 28 having a periodic unevenness on the side wall as shown in FIG. 2C was formed.

Next, as a third step, an optical guide layer is formed by using a vapor phase growth method.
20, the active layer 19, and the p-type conductive layer 18 were sequentially stacked. As a property peculiar to the vapor phase growth method, when the growth in the step shape is performed, the growth on the side wall and the bottom surface simultaneously proceeds. Utilizing the properties peculiar to this vapor phase growth method, an optical waveguide region (active region) 24 formed of a part of the optical guide layer 20 and the active layer 19 was formed. In addition,
Prior to this vapor phase growth, after removing the Ti mask layer 26,
The semiconductor layer was finely etched using a bromine methanol solution to remove the damage on the surface of the shape formed by the reactive ion etching.

Finally, as a fourth step, electrodes were formed on the thus-formed semiconductor wafer by using a normal photolithography technique. First, after removing the SiO ₂ mask layer 27, SiO on the front ₂
Layer 15 and resist 29 were formed. Then, after forming an n-type electrode pattern on the resist 29 by a photolithography method, the SiO ₂ layer 15 and the semi-insulating cap layer 13 are removed using the resist 29 as a mask to form the n ⁺ -type conductive layer 12. Opened the window. Further, after depositing the n-type electrode 14, unnecessary resist 29 and electrode metal were removed by using the lift-off method (see FIG. 2 (d)).

Then, a resist 31 is formed on the front surface, a p-type electrode pattern is formed on the resist 31 by a photolithography method, and then the SiO ₂ layer 15 is removed using the resist 31 as a mask to form a p-type conductive layer 18. Opened the window. Furthermore, p-type electrode
After depositing 17, use the lift-off method to useless resist
31 and the electrode metal were removed (see FIG. 2 (e)).

The above first to fourth steps are all methods suitable for mass production. In particular, vapor phase growth in the first and third steps has high reproducibility and high uniformity. Further, the electron beam exposure and the reactive ion etching in the second step have high reproducibility and high uniformity. Therefore, it was found that the characteristics of the semiconductor laser manufactured by such a method have little variation and can be obtained with a high yield.

Further, since the width of the active region 24 is determined only by the layer thickness of the n ^{+ type} conductive layer 12, it does not depend on the accuracy of photolithography and chemical etching. Therefore, the size of the active region 24 could be controlled with high accuracy by using vapor phase growth with high controllability.

In the above embodiment, the bandgap of the active region 24 is 0.8 eV, so 1.5
A laser that oscillates at 5 μm was obtained.
It can be set to oscillate at any wavelength from 1.3 μm to 1.65 μm.

Although InGaAsP / InP-based crystals were used in the above-mentioned examples, AlGsAs /
Other crystals such as GaAs and AlInGaP / GaAs can be used.

〔The invention's effect〕

The semiconductor laser according to the present invention is capable of high-speed modulation exceeding 10 Gb / s while maintaining an axial single mode, oscillates with extremely high efficiency, and obtains a transverse single mode in a waveguide region. The size can be obtained with good reproducibility, so that the oscillation characteristics with high uniformity can be obtained.

Further, according to the manufacturing method of the present invention, such a semiconductor laser can be manufactured with high yield and high mass productivity.

[Brief description of drawings]

FIG. 1 is a sectional view of a semiconductor laser which is an embodiment of the present invention, FIG. 2 is a manufacturing process diagram of a semiconductor laser for explaining a method of manufacturing a semiconductor laser which is an embodiment of the present invention, and FIG. FIG. 6 is a sectional view of a semiconductor laser for explaining a conventional example. 10 …… Semi-insulating semiconductor substrate 11 …… Semi-insulating buffer layer 12 …… n ^{+ type} conductive layer 13 …… Semi-insulating cap layer 14 …… N-type electrode 15 …… Insulating film 16 …… Active region 17… ... p-type electrode 18 ... p-type conductive layer 19 ... active layer 20 ... optical guide layer 21 ... n-type cladding layer 22 ... p-type cladding layer 23 ... Zn diffusion layer 24 ... optical waveguide region (active Area) 25 …… p ^{+ type} conductive layer 26 …… Ti mask layer 27 …… SiO ₂ mask layer 28 …… Diffraction grating 29,31 …… Resist 30 …… Electron beam resist

Claims (2)

[Claims] 1. A first clad and a second clad to the left and right of an active region composed of an active layer laminated in a direction perpendicular to a substrate surface and an optical guide layer having a forbidden band width larger than the forbidden band width of the active layer. Is provided on a semi-insulating semiconductor substrate, and the first cladding part is composed of two semi-insulating semiconductor layers having a forbidden band width larger than the forbidden band width of the light guide layer. The second cladding portion has a sandwiched structure, has a conductivity type opposite to that of the semiconductor layer, and has a forbidden band width larger than the forbidden band width of the active layer. A semiconductor laser having periodical unevenness at an interface between a guide layer and the first cladding portion, and electrodes being provided on the first cladding portion and the second cladding portion, respectively. 2. A step of forming a laminated structure in which a conductive layer of a first conductivity type is sandwiched between semi-insulating semiconductor layers having a band gap larger than the band gap of the conductive layer on a semi-insulating semiconductor substrate.
Forming a step shape in the laminated structure having a side surface having periodic irregularities and having a depth reaching the semi-insulating semiconductor substrate; and forming a forbidden band width smaller than the forbidden band width of the conductive layer on the side surface. A step of forming an optical guide layer having the same and an active layer having a forbidden band width smaller than the forbidden band width of the light guide layer, and thereafter forming a clad layer of the second conductivity type so as to fill the step shape. A method for manufacturing a characteristic semiconductor laser.