JP2015220323A - Semiconductor optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lateral implantation laser on an InP board preventing deterioration of the characteristics of laser by capable of suppressing degradation of optical confinement even when the width of an active layer is set to be less than 1 μm.SOLUTION: A semiconductor optical device 1 is so configured as to form an active layer 4 and an embedded waveguide structure on a semiconductor board 2, subject embedded layers 7 and 8 at both sides of the active layer to impurity doping, and laterally inject a current into the active layer in the lateral direction (width direction), where the thickness of an upper optical carrier separation and confinement (SCH) layer 5 is larger than the thickness of a lower optical carrier SCH layer 3. The thickness of the upper optical carrier SCH layer and the thickness of the lower optical carrier SCH layer are set so that the optical confinement of the active layer is maximum.

Description

  The present invention relates to a light source for an optical transmitter for optical communication. More specifically, the present invention relates to a semiconductor optical device applicable to a semiconductor laser for a light source.

  Due to the explosive increase in the amount of network traffic accompanying the spread of the Internet, optical fiber transmission continues to increase in speed and capacity. Semiconductor lasers have continued to develop as basic light source devices that support optical fiber communications. Direct modulation lasers that generate intensity-modulated signals by modulation of current intensity have simple laser configurations and low power consumption, so they are used as low-cost optical transmitters used in access networks and the like. Yes.

  In a conventional semiconductor laser, a lower cladding layer, an active layer, and an upper cladding layer are formed on a semiconductor substrate, impurity doping is performed on the cladding layers above and below the active layer, and the longitudinal direction (direction perpendicular to the substrate surface) ) Had a structure for injecting current. In contrast, Namizaki devised a so-called lateral injection laser that injects a current in the active layer in a horizontal direction (a direction parallel to the substrate surface) (Non-patent Document 1). The lateral injection laser performs impurity doping on the cladding layer next to the active layer and injects a current in the lateral direction (width direction) of the active layer. In the lateral injection laser, the cross section of the active layer is generally formed by a flat structure that is long in a direction parallel to the substrate. When an active layer having the same structure is used for a laser, the parasitic capacitance of the element is lower in the lateral injection structure than in the vertical injection structure. Therefore, the laser having the lateral injection structure is more suitable for high-speed modulation operation because it responds more quickly to signals. In addition, since the lateral injection laser can form an electrode for current injection on the surface of the laser, it is also an excellent feature that it is suitable for integration of electronic devices and modularization.

  Specifically, Namizaki et al. Realized a lateral injection type semiconductor laser configured on a GaAs substrate, and Kawamura et al. Also realized a lateral injection laser configured on an InP substrate (Non-patent Document 2). Hereinafter, first, a more specific configuration of the lateral injection laser will be described.

  FIG. 6 is a diagram showing the structure of a waveguide for a lateral injection laser according to the prior art. A waveguide structure 100 is shown in which a waveguide used for a resonator portion of a laser is cut by a plane perpendicular to the light reciprocation direction. In the waveguide structure 100, an active layer 104 is formed on a semi-insulating InP substrate 102, and a lower light-carrier separation confinement (SCH) layer 103 and an upper SCH are formed above and below the active layer 104, respectively. The layer 105 is formed in a vertically symmetrical structure. Further, an InP layer 106 is formed on the upper SCH layer 105. The sides of the active layer 104 are embedded with InP on both the left and right sides. In FIG. 6, the buried layer 107 on the left side of the active layer 504 is subjected to n-type doping for current injection, and the buried layer 108 on the right side of the active layer 104 is subjected to p-type doping. On the buried layer 107 and the buried layer 108, InGaAs contact layers 109a and 109b for current injection are formed. Furthermore, electrodes 110a and 110b for current injection are formed on the contact layers 109a and 109b, respectively.

  In the study by Kawamura et al., The thickness of the InP cladding layer 503b above the active layer 504 is 1.5 μm, which is the same structure as that of the conventional vertical injection laser. In recent years, Shindo et al. Realized a thin film lateral injection laser composed of a thin active layer having a thickness of less than 400 nm and a thin InP layer on an InP substrate (Non-patent Document 3). As already described, in the case of the lateral injection laser structure, there is an advantage that the parasitic capacitance of the element can be suppressed by reducing the thickness of the cladding layers above and below the active layer. Shindo et al. Realized a wide modulation band up to 5 GHz by adopting this thin film structure in a lateral injection laser.

H. Namizaki et al., Journal of Applied Physics, vol. 45, pp. 2785-2786 (1974) Y. Kawamura et al., Electronics Letters, vol. 29, pp. 102-104 (1993) T. Shindo et al., Optics Express, vol. 19, pp. 1884-1891 (2011)

  The lateral injection laser has a structure for injecting current in the lateral direction (width direction) of the active layer, and modulates the injected current in the case of direct modulation operation. Therefore, from the viewpoint of the parasitic capacitance of the element, it is desirable that the layers above and below the active layer be as thin as possible as described above. Further, since the response becomes slow as the carrier travel distance in the direction parallel to the substrate increases, it is desirable that the width of the active layer can be made as narrow as possible from the viewpoint of high-speed modulation. However, in the conventional thin film laser, when the width of the active layer 104 is narrowed, the light in the active layer leaks greatly to the substrate 102 side, and there is a problem that sufficient light confinement in the active layer cannot be performed.

FIG. 7 is a diagram showing the calculation result of the electric field distribution of the waveguide mode of the one-dimensional slab structure in the conventional lateral injection laser. The horizontal axis indicates the position in the thickness direction of the substrate, and the vertical axis indicates the refractive index and the electric field strength. The horizontal curve distribution in the graph corresponds to each layer structure in the upper thickness direction. The active layer 104 was calculated as a quantum well having a light emission wavelength of 1.55 μm composed of 14 layers having a well layer thickness of 6 nm and a barrier layer thickness of 9 nm. The InGaAsP SCH layers 103 and 105 having a band gap wavelength of 1.2 μm are formed above and below the active layer, and the thicknesses of the upper and lower SCH layers are set to 65 nm, which are the same length. The thickness of the InP layer above the active layer 104 (the right side in FIG. 7) was 100 nm.

  In a vertical injection type semiconductor laser, in order to efficiently confine carriers and light, it is common to provide two SCH layers above and below an active layer and provide a vertically symmetrical SCH structure. That is, the thickness of the lower SCH layer and the thickness of the upper SCH layer are the same. In the vertical injection type, the upper cladding layer can be made sufficiently thick, and the electric field distribution of the waveguide mode of light with respect to the direction perpendicular to the substrate is vertically symmetric, so the center position of the active layer and the center position of the electric field distribution Match.

  However, when the structure of the vertical injection type laser is applied as it is to the lateral injection type laser using the thin film structure shown in FIG. 6, the electric field distribution of the optical waveguide mode and the optical confinement are as follows. Strongly affected by air. As is clear from FIG. 7, the electric field distribution in the waveguide mode greatly spreads toward the substrate 102, and the center position 113 of the active layer does not coincide with the center position 114 of the electric field distribution, so that the optical confinement is lowered. This tendency of the electric field distribution to shift toward the substrate side becomes more remarkable as the width of the active layer 104 becomes narrower.

  FIG. 8 is a diagram illustrating the dependence of the optical confinement coefficient on the active layer width in a two-dimensional buried waveguide. Here, the optical confinement factor is calculated in consideration of two dimensions in the width direction of the active layer and the thickness direction of the active layer. The optical confinement factor is generally defined as the ratio of the electric field strength in the quantum well of the active layer to the total electric field strength. It is obtained by calculating the electric field distribution of the propagation mode of the two-dimensional cross section using a finite difference method or the like and numerically calculating the entire electric field strength and the electric field strength in the active layer. In the graph of FIG. 8, the optical confinement coefficient for the TE mode was calculated. The prerequisite waveguide structure is the same as the structure condition calculated in FIG.

  As shown in FIG. 8, the optical confinement factor is greatly reduced with the reduction of the active layer width, the degree of optical confinement in the active layer is lowered, and the laser characteristics are deteriorated. Due to the influence of the reduction of the optical confinement factor, a lateral injection thin film laser having a width of the active layer 104 of less than 1 μm has not been realized in the lateral injection laser on the InP substrate. Therefore, in the lateral injection laser, it has been desired to suppress a decrease in optical confinement even when the width of the active layer 104 is narrowed.

  The present invention has been made in view of such problems, and an object of the present invention is to realize a semiconductor laser capable of high efficiency and high speed operation even when the width of the active layer is narrowed. It is in.

In order to achieve the above object, the present invention includes a semiconductor substrate, an active layer formed on the semiconductor substrate, and buried layers on both sides of the active layer. A waveguide structure is formed, one of the buried layers is doped with a first type of impurity, the other of the buried layers is doped with a second type of impurity, the one buried layer and the other In the semiconductor optical device in which a structure for performing current injection in the lateral direction is formed between the buried layers, the waveguide structure is in contact with the active layer on the substrate side of the active layer. A lower optical carrier isolation and confinement layer having a first thickness and the active layer sandwiched between the lower optical carrier isolation and confinement layer and facing the lower optical carrier isolation and confinement layer; 2nd larger than Further comprising an upper beam carrier separate confinement layers having a thickness of a semiconductor optical device according to claim.
According to a second aspect of the present invention, in the semiconductor optical device according to the first aspect, a clad layer is further provided on the upper optical carrier separation confinement layer.

  According to a third aspect of the present invention, in the semiconductor optical device according to the first or second aspect, the first thickness and the second thickness are set so that optical confinement of the active layer is maximized. It is characterized by.

  According to a fourth aspect of the present invention, in the semiconductor optical device according to any one of the first to third aspects, the semiconductor substrate is a semi-insulating InP substrate, and the active layer and the optical carrier separation and confinement layer are respectively InGaAsP or The buried layer is made of InP, the first type corresponds to p-type impurity doping, and the second type corresponds to n-type impurity doping.

  According to a fifth aspect of the present invention, in the semiconductor optical device according to any one of the second to fourth aspects, the upper cladding layer is made of InP.

  According to a sixth aspect of the present invention, in the semiconductor optical device according to any one of the first to fifth aspects, the sum of the thicknesses of the active layer, the optical carrier separating and confining layer, and the upper cladding layer is between 350 nm and 550 nm. It is characterized by that.

  According to a seventh aspect of the present invention, in the semiconductor optical device according to any one of the first to sixth aspects, a thickness ratio of the upper optical carrier separation confinement layer and the lower optical carrier separation confinement layer is 6: 4 or more, The upper optical carrier separation and confinement layer is thicker.

  According to an eighth aspect of the present invention, there is provided a semiconductor laser having the waveguide structure of the semiconductor optical device according to any one of the first to seventh aspects.

  The invention according to claim 9 is characterized in that it has the waveguide structure of the semiconductor optical device according to any one of claims 1 to 8, and comprises a surface diffraction grating formed above the active layer. This is a distributed feedback semiconductor laser.

  As described above, according to the configuration of the semiconductor optical device of the present invention, it is possible to realize a semiconductor laser capable of high efficiency and high speed operation even when the width of the active layer is narrowed.

FIG. 1 is a diagram showing a waveguide and an overall structure of a lateral injection laser according to the present invention. FIG. 2 is a diagram showing the calculation result of the optical confinement coefficient of the active layer in the one-dimensional slab structure when the thicknesses of the two SCH layers are changed. FIG. 3 is a diagram comparing and comparing the relationship between the optical confinement factor of the active layer and the active layer width in the lateral injection lasers having the structures of the prior art and the semiconductor optical device of the present invention. FIG. 4 is a diagram showing the ratio of the optical confinement factor between the transverse injection lasers according to the structures of the prior art and the semiconductor optical device of the present invention. FIG. 5 is a diagram comparing the calculated values of the threshold currents in the distributed feedback laser having the structures of the prior art and the semiconductor optical device of the present invention. FIG. 6 is a diagram showing the structure of a waveguide for a lateral injection laser according to the prior art. FIG. 7 is a diagram showing the calculation result of the electric field distribution of the waveguide mode of the one-dimensional slab structure in the conventional lateral injection laser. FIG. 8 is a diagram illustrating the dependence of the optical confinement coefficient on the active layer width in a two-dimensional buried waveguide. FIG. 9 is a comparison diagram of confinement factors when the active layer thickness H _act is 100 nm and the core layer thickness, the upper InP cladding layer thickness, and the SCH layer thickness are combined. FIG. 10 is a comparison diagram of confinement factors when the active layer thickness H _act is 200 nm and the core layer thickness, upper InP cladding layer thickness, and SCH layer thickness are combined.

  In the semiconductor optical device of the present invention, an active layer and a buried waveguide structure are formed on a semiconductor substrate, impurity doping is performed on the buried layers on both sides of the active layer, and current is applied to the active layer in the lateral direction (width direction). In the structure in which the injection is performed, the thickness of the upper optical carrier separation confinement (SCH) layer is configured to be larger than the thickness of the lower optical carrier SCH layer. Further, the thickness of the upper optical carrier SCH layer and the thickness of the lower optical carrier SCH layer are set so that the optical confinement of the active layer is maximized. A semiconductor laser can be configured by forming a structure in which impurity is doped in the buried layer next to the active layer and current is injected in the lateral direction in the active layer. In the semiconductor optical device described above, a distributed feedback semiconductor laser can be configured by forming a diffraction grating on the surface of the active layer. Furthermore, the present invention can be applied to an electroabsorption modulator by applying a reverse bias to the active layer. Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1A is a diagram showing the structure of a waveguide in which the semiconductor optical device of the present invention is applied to a lateral injection laser. FIG. 1A shows a waveguide structure 1 in which a waveguide used for a laser resonator is cut by a plane perpendicular to the reciprocating direction of light in the resonator. The waveguide structure 1 includes an InGaAsP optical carrier separation confinement (hereinafter referred to as a lower SCH layer) layer 3 having a band gap wavelength of 1.2 μm and an InGaAsP active layer 4 having an emission wavelength of 1.55 μm on a semi-insulating InP substrate 2. It is formed sequentially. The active layer 4 is formed of a quantum well composed of 14 layers having a well layer thickness of 6 nm and a barrier layer thickness of 9 nm. An InGaAsP upper SCH layer 5 having a band gap wavelength of 1.2 μm is formed on the active layer 4. Further, an InP layer 6 having a thickness of 100 nm is formed on the upper SCH layer 5 above the active layer 4.

  In the lateral injection laser waveguide using the structure of the semiconductor optical device of the present invention, the thickness of the upper SCH layer 5 is set to be greater than the thickness of the lower SCH layer 3, and here the thickness of the upper SCH layer 5 is set. 110 nm, and the thickness of the lower SCH layer 3 is 20 nm. The configuration of the prior art lateral injection laser is in contrast to the configuration in which the two SCH layers have the same thickness.

On the surface of the InP layer 6, a diffraction grating having a Bragg wavelength of 1.55 μm made of InP and air is formed by etching InP. Both sides of the active layer 4 are filled with InP with different types of doping. That is, for current injection, a Si n-type doping layer 7 having a doping concentration of 1 × 10 ¹⁸ cm ⁻³ is formed on the left side of the active layer 4 in FIG. A p-type doping layer 8 of 10 ¹⁸ cm ⁻³ of Zn is formed. On the buried layers 7 and 8, InGaAs contact layers 9a and 9b for current injection are formed, respectively, and n-type doping and p-type doping with a doping concentration of 1 × 10 ¹⁹ cm ⁻³ are performed, respectively. Furthermore, current injection electrodes 10a and 10b are formed on the contact layer regions 9a and 9b, respectively.

  Therefore, the semiconductor optical device of the present invention has a waveguide structure including the semiconductor substrate 2, the active layer 4 formed on the semiconductor substrate, and the buried layers 7 and 8 on both sides of the active layer. A first type of impurity doping is performed on one of the buried layers, a second type of impurity doping is performed on the other of the buried layers, and between the one buried layer and the other buried layer, In the semiconductor optical device in which a current injection structure is formed in the active layer in the lateral direction, the waveguide structure is formed on the substrate side of the active layer in contact with the active layer, and has a first thickness. A lower optical carrier isolation / confinement layer 3 having a thickness, and a second optical carrier sandwiched between the active layer and in contact with the active layer, opposite the lower optical carrier isolation / confinement layer, and having a second thickness greater than the first thickness Top with thickness It is implemented as having a carrier separate confinement layer 5.

  FIG. 1B is a bird's-eye view of a lateral injection laser to which the structure of the semiconductor optical device of the present invention is applied, viewed from obliquely above. In the lateral injection laser to which the structure of the semiconductor optical device of the present invention is applied, the surface diffraction grating 11 is formed in the region between the electrodes 10a and 10b on the uppermost InP layer, thereby forming a resonator. This is a distributed feedback laser. Anti-reflective coating is applied to the laser output end faces at both ends of the resonator. In producing the lateral injection laser of the present invention, metal organic vapor phase epitaxy (MOVPE) is used for crystal growth, and a general semiconductor laser such as wet etching or dry etching is used for producing a laser waveguide structure and a diffraction grating. The method can be used. The buried doping layers 7 and 8 for current injection on the left and right sides of the active layer 4 can be formed by burying regrowth with n-type doped InP and p-type doped InP, respectively. Further, after the active layer 4 is formed, intrinsic InP may be buried and regrown, and then a dopant may be formed by a technique such as ion implantation or thermal diffusion. Next, an SCH layer design method unique to the present invention will be described.

  FIG. 2 is a diagram showing a calculation result of the optical confinement coefficient of the active layer in the one-dimensional slab structure when the thicknesses of the two SCH layers are changed simultaneously. In the configuration of FIG. 1, the total thickness of the upper SCH layer and the lower SCH layer is fixed to a constant value of 130 nm, and the activity in the one-dimensional slab structure corresponding to the case where the thickness of the upper SCH layer 5 is changed. The value of the optical confinement factor of the layer is shown. That is, the optical confinement factor was calculated in consideration of only the thickness direction of the substrate. The laser having the structure of the prior art has a structure in which the two SCH layers have the same thickness and are vertically symmetrical, and the lower SCH layer 3 and the upper SCH layer 5 both have a structure of 65 nm. In the situation corresponding to this prior art in FIG. 2 (width is 65 nm), the electric field distribution of the waveguide mode of light spreads to the substrate side as described above, and the center position of the electric field distribution and the center position of the active layer are one. Since it did not do, it turns out that the optical confinement factor has fallen considerably from the peak.

  On the other hand, when the thickness of the upper SCH layer 5 is increased toward the right in the curve of FIG. 2 and the thickness of the lower SCH layer is decreased, the value of the optical confinement factor increases, and the thickness is around 110 nm. Is the largest. At this time, the lower SCH layer 3 is 20 nm, and the upper SCH layer 5 is 110 nm. In the region near the maximum value of the optical confinement coefficient, the center position of the active layer and the center position of the electric field distribution of the waveguide mode are almost the same. Comparing the prior art vertically symmetric structure with the structure of the present invention in which the thickness of the upper SCH layer 5 is set larger than the thickness of the lower SCH layer 3, it is clear that the optical confinement value is increased by the present invention. It is. Therefore, in the semiconductor optical device of the present invention, the first thickness of the lower optical carrier isolation and confinement layer 3 and the second thickness of the upper optical carrier isolation and confinement layer 5 are set so that the optical confinement of the active layer is maximized. Will be set.

  FIG. 3 is a diagram comparing the optical confinement coefficient of the active layer of the buried structure while changing the width of the active layer in the lateral injection laser having the structures of the prior art and the semiconductor optical device of the present invention. In the prior art structure, the upper SCH layer and the lower SCH layer are both 65 nm. In the structure of the present invention, the upper SCH layer was 110 nm and the lower SCH layer was 20 nm. As is apparent from FIG. 3, the structure of the present invention has a larger optical confinement factor in the entire region of the width of the active layer. As will be described below, the configuration of the semiconductor optical device of the present invention is particularly effective for a semiconductor optical device having a narrow active layer width.

  FIG. 4 is a diagram showing the waveguide width dependence of the ratio of the optical confinement factor between the conventional technology and the lateral injection laser having the structures of the semiconductor optical device of the present invention. From each calculation result of the optical confinement coefficient shown in FIG. 3, the optical confinement ratio (coefficient of the structure of the present invention / coefficient of the prior art structure) was obtained. In either case, the optical confinement factor decreases as the active layer width becomes narrower, but the reduction of the optical confinement factor can be suppressed when compared with the configuration of the present invention and the structure of the prior art. I understand that. In particular, in a lateral injection laser device having an active layer width of 0.6 μm, an improvement of 1.3 times is observed, and it can be seen that the configuration of the semiconductor optical device of the present invention is more effective as the active layer width is narrower. .

FIG. 5 is a diagram comparing the calculated values of the threshold current of the distributed feedback laser with each structure of the prior art and the semiconductor optical device of the present invention. The horizontal axis represents the resonator length (μm), and the vertical axis represents the threshold current (mA). The coupling constant of the diffraction grating was 100 cm ⁻¹ . The effect of improving the threshold due to the structure of the semiconductor optical device of the present invention is clear, and the effect of improving the threshold current is particularly large when the resonator length is short. As described above, according to the present invention, a highly efficient lateral injection laser can be realized.

Next, a more specific configuration example when the material of the active layer is different is shown. In the waveguide structure shown in FIG. 1, the active layer 4 has a quantum well structure or a bulk structure made of an InGaAsP material having an emission wavelength of 1.55 μm. Similar to the first embodiment, the lower SCH layer 3, the upper SCH layer 5, and the upper InP cladding layer 6 having a band gap wavelength of 1.2 μm are used. The core layer thickness of the active layer 4 and the upper and lower SCH layers is H _core , the active layer thickness is H _act , the upper InP cladding layer thickness is H _InP , and the SCH layer thickness of the upper and lower SCH layers combined In various combinations where the thickness is H _SCH , the optimum thickness of the upper SCH layer was studied. That is, in each of the following graphs, H _SCH indicates the sum of the thickness of the upper SCH layer 5 and the thickness of the lower SCH layer 3. The optimum value of the thickness of the upper SCH is examined while keeping the SCH layer thickness H _SCH constant.

FIG. 9 shows the case where the active layer thickness H _act is 100 nm, the core layer thickness H _core , the upper InP cladding layer thickness H _InP , and the SCH layer thickness H _SCH are combined. It is the figure which compared the confinement factor. 9A shows the case where the core layer thickness H _core = 250 nm and the SCH layer thickness H _SCH = 150 nm, and FIG. 9B shows the core layer thickness H _core = 350 nm and the SCH layer thickness. FIG. 9C shows the case where H _SCH = 250 nm, and FIG. 9C shows the case where the core layer thickness H _core = 450 nm and the SCH layer thickness H _SCH = 350 nm.

FIG. 10 shows the case where the active layer thickness H _act is 200 nm, the core layer thickness H _core , the upper InP cladding layer thickness H _InP , and the SCH layer thickness H _SCH are combined. It is the figure which compared the confinement factor. 10A shows the case where the core layer thickness H _core = 250 nm and the SCH layer thickness H _SCH = 50 nm, and FIG. 10B shows the core layer thickness H _core = 350 nm and the SCH layer thickness. In the case of H _SCH = 150 nm, FIG. 10C shows the case where the core layer thickness H _core = 450 nm and the SCH layer thickness H _SCH = 250 nm.

  In each graph, the horizontal axis indicates the thickness of the upper SCH layer. The left end of the horizontal axis corresponds to a structure in which the upper SCH layer is 0 nm and the lower SCH thickness is maximum, and the right end of the horizontal axis corresponds to a structure in which the upper SCH layer is maximum and the lower SCH layer is 0 nm. The center of the horizontal axis of each graph corresponds to the case where the SCH layer is vertically symmetrical.

  From FIG. 9 and FIG. 10, it is clear that the configuration that maximizes the optical confinement factor is the case where the thickness of the upper SCH layer is thicker than that of the lower SCH layer in any structure. When the thickness of the core layer is as thin as 250 nm (in the case of FIGS. 9 and 10A), the optical confinement factor is significantly reduced when the upper InP thickness is 100 nm or less. Therefore, in order to ensure optical confinement, the sum of the thickness of the core layer and the thickness of the upper InP cladding layer is preferably at least 350 nm or more.

  Further, as can be seen in the case where the thickness of the core layer is 350 nm and 450 nm (in the case of FIGS. 9 and 10 (b) and (c)), the sum of the thickness of the core layer and the thickness of the upper cladding layer is When it becomes approximately 550 nm or more, the maximum value of the optical confinement coefficient starts to decrease. Therefore, in the configuration of the quantum well structure or the bulk structure made of InGaAsP material, the thickness of the core layer and the thickness of the upper cladding layer are set in the range of 350 nm to 550 nm, and the thickness of the upper SCH layer is set to be larger than that of the lower SCH layer. It is preferable to increase the thickness. The ratio of the thickness of the upper SCH layer and the lower SCH layer is 6: 4 or more, and the effect is obtained when the upper SCH layer is thicker.

In each of the embodiments described above, an InGaAsP material is used for the active layer, but it goes without saying that other optical semiconductor material systems such as an InAlGaAs material can be applied. In the above-described embodiment, the distributed feedback semiconductor laser that realizes the oscillation operation by the diffraction grating formed above the active layer has been described as an example. However, other examples such as a Fabry-Perot laser and a distributed Bragg reflection laser have been described. The present invention can also be applied to a semiconductor laser using an oscillator having a configuration. Regarding the shape of the diffraction grating, the upper InP was etched to obtain an InP and air layer, but a protective film such as SiN or SiO ₂ was formed on the upper part to form a diffraction grating made of InP and a protective film. It may be formed. The same effect can be obtained by forming a diffraction grating of SiN or SiO ₂ on InP.

  In each of the above-described embodiments, the structure of the semiconductor optical device of the present invention is applied to a semiconductor laser. However, since optical confinement in the waveguide structure can be improved, an electric field can be applied by applying a reverse bias to the active layer. Obviously, even if an absorption modulator is realized, the same improvement in the characteristics of electroabsorption can be obtained.

  As described above, with the configuration of the semiconductor optical device of the present invention, it is possible to realize a semiconductor laser capable of high efficiency and high speed operation even when the width of the active layer is narrowed.

  The present invention is generally applicable to communication systems. In particular, it can be used for an optical transmitter of an optical communication system.

DESCRIPTION OF SYMBOLS 1,100 Waveguide structure 2,102 Substrate 3,103 Lower SCH layer 4,104 Active layer 5,105 Upper SCH layer 6,106 InP layer 7,8,107,108 Embedded InP layer
9a, 9b, 109a, 109b Contact layer 10a, 10b, 110a, 110b Electrode 11 Diffraction grating

Claims (9)

A waveguide structure comprising a semiconductor substrate, an active layer formed on the semiconductor substrate, and buried layers on both sides of the active layer is formed,
A first type of impurity doping is performed on one of the buried layers, a second type of impurity doping is performed on the other of the buried layers, and between the one buried layer and the other buried layer, In a semiconductor optical device in which a structure for injecting current in a lateral direction is formed in an active layer, the waveguide structure is:
A lower optical carrier isolation and confinement layer formed on and in contact with the active layer on the substrate side of the active layer and having a first thickness;
An upper optical carrier isolation and confinement layer having a second thickness larger than the first thickness, which is formed in contact with the active layer so as to face the lower optical carrier isolation and confinement layer with the active layer interposed therebetween. A semiconductor optical device comprising the semiconductor optical device.   The semiconductor optical device according to claim 1, further comprising a cladding layer on the upper optical carrier separation confinement layer.   3. The semiconductor optical device according to claim 1, wherein the first thickness and the second thickness are set so that optical confinement of the active layer is maximized.   The semiconductor substrate is a semi-insulating InP substrate, the active layer and the optical carrier separation and confinement layer are each made of InGaAsP or InGaAlAs, the buried layer is made of InP, and the first type is a p-type impurity. 4. The semiconductor optical device according to claim 1, wherein the semiconductor optical device corresponds to doping, and the second type corresponds to n-type impurity doping. 5.   5. The semiconductor optical device according to claim 2, wherein the upper cladding layer is made of InP.   6. The semiconductor optical device according to claim 1, wherein the sum of the thicknesses of the active layer, the optical carrier separation confinement layer, and the upper cladding layer is between 350 nm and 550 nm.   7. The thickness ratio of the upper optical carrier separation confinement layer and the lower optical carrier separation confinement layer is 6: 4 or more, and the upper optical carrier separation confinement layer is thicker. The semiconductor optical device described in 1.   A semiconductor laser having the waveguide structure of the semiconductor optical device according to claim 1.   9. A distributed feedback semiconductor laser having the waveguide structure of the semiconductor optical device according to claim 1, further comprising a surface diffraction grating formed above the active layer.

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