Patent Publication Number: US-2023139692-A1

Title: Semiconductor Optical Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national phase entry of PCT Application No. PCT/JP2020/014508, filed on Mar. 30, 2020, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a semiconductor optical device with a heterostructure. 
     BACKGROUND 
     There is recently being requested lower power consumption on optical devices for short-distance optical communication, due to increasing communication volume in data centers and the like. To handle this request, there have been developed optical devices having a buried heterostructure formed in a stacked structure in which a plurality of thin semiconductor layers are stacked (Non-Patent Literatures 1 to 5). The buried heterostructure is a structure in which a semiconductor (active layer) having larger refractive index and bandgap is sandwiched by semiconductors having smaller refractive indices and bandgaps than those of the active layer in the up-down and right-left directions. This structure can improve a coefficient, of light confinement to the active layer, which largely contributes to various kinds of performance of a semiconductor optical device. 
     A device having the buried heterostructure in the thin film structure as above is characterized by a contrivance that power consumption is maintained to be low by using a semiconductor multilayer structure typically with about 250 nm to 500 nm of thickness to make the volume of the semiconductor active layer small. Moreover, for the active layer, there is employed a multiple quantum well (MQW) structure excellent in carrier coupling efficiency. 
     In the device having such a configuration, the semiconductor structure is made thin in order to attain stronger light confinement by making a layer (cladding) different in refractive index closer to a region which is to be a core. As configurations to perform electric field application and current injection to the active layer, this main device has a vertical pin structure in which semiconductor layers above on and below on the active layer as viewed from the substrate side are of the p-type and the n-type, and a lateral pin structure in which semiconductor layers to the right and the left of the active layer are of the p-type and the n-type. Moreover, in order to perform voltage application, there can also be used an n-p-i-n structure in which semiconductors above on and below on an i-type active layer are of the p-type and the n-type and an n-type semiconductor is formed on the opposite side to the p-type semiconductor as viewed from the active layer. 
     The vertical pin structure allows light to be guided in the active layer not to be affected by electrodes by forming a p-type layer arranged on the upside of the active layer to be thick in order to make an electrode arranged on the upper portion of the active layer separate from the active layer. Since the vertical pin structure can allow layers, for example, of semi-insulating semiconductor to be arranged on the right and the left of the active layer, it can be a structure small in current leakage. 
     On the other hand, the lateral pin structure can allow the electrode to be arranged with a displacement from the upper portion of the active layer without the thick p-type layer formed on the upside of the active layer, and provides a configuration basically allowing light guided in the active layer to be scarcely affected by electrodes. Moreover, the lateral pin structure does not need formation of electroconductive layers above on and below on the active layer, can allow arrangement of layers having lower refractive indices than the active layer (semiconductor) which is composed of an insulating material or the like, and can realize higher light confinement to the active layer. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature 1: S. Matsuo, T. Fujii, K. Hasebe, K. Takeda, T. Sato, and T. Kakitsuka, “Directly modulated buried heterostructure DFB laser on SiO 2 /Si substrate fabricated by regrowth of InP using bonded active layer”, OPTICS EXPRESS, vol. 22, no. 10, pp. 12139-12147, 2014. 
         Non-Patent Literature 2: T. Okamoto et al., “Optically Pumped Membrane BH-DFB Lasers for Low-Threshold and Single-Mode Operation”, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, vol. 9, no. 5, pp. 1361-1366, 2003. 
         Non-Patent Literature 3: S. Matsuo, K. Takeda, T. Sato, M. Notomi, A. Shinya, K. Nozaki, H. Taniyama, K. Hasebe, and T. Kakitsuka, “Room-temperature continuous-wave operation of lateral current injection wavelength-scale embedded active-region photonic-crystal laser”, OPTICS EXPRESS, vol. 20, no. 4, pp. 3773-3780, 2012. 
         Non-Patent Literature 4: S. Matsuo1, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato, Y. Kawaguchi and M. Notomi, “High-speed ultracompact buried heterostructure photonic-crystal laser with 13 fJ of energy consumed per bit transmitted”, NATURE PHOTONICS, vol. 4, pp. 648-654, 2010. 
         Non-Patent Literature 5: K. Hasebe, T. Sato, K. Takeda, T. Fujii, T. Kakitsuka, and S. Matsuo, “High-Speed Modulation of Lateral p-i-n Diode Structure Electro-Absorption Modulator Integrated With DFB Laser”, Journal of Lightwave Technology, vol. 33, no. 6, pp. 1235-1240, 2015. 
       
    
     SUMMARY 
     Technical Problem 
     However, the vertical pin structure cannot allow layers of an insulating material to be arranged on the upside and the downside of the active layer, which disables light confinement to the active layer to be very high. 
     Moreover, while in semiconductor optical devices of this type, semiconductor layers are formed to be on and in contact with the lower surface and the upper surface of the active layer in order to restrain an interface state from arising, the lateral pin structure needs to have the active layer and the upper and lower semiconductor layers sequentially formed in the same process, and it is difficult to make the upper and lower semiconductors semi-insulating. Therefore, there is a problem on the lateral pin structure that the semiconductor layers on the lower surface and the upper surface of the active layer provide leak paths of current. Moreover, there is a problem on the lateral pin structure that the interfaces between the semiconductor layers above on and below on the active layer and the low refractive index layers also provide current leak paths. 
     Like these, there have been a problem on the conventional technologies that it is not easy to realize high light confinement to the active layer without a leak path of current formed. 
     Embodiments of the present invention are devised to solve some problem(s) as above, and an object thereof is to realize high light confinement to an active layer without a leak path of current formed. 
     Means for Solving the Problem 
     There is provided a semiconductor optical device according to embodiments of the present invention, including: a first low refractive index layer having a lower refractive index than semiconductor, the first low refractive index layer being formed on a substrate; a first semiconductor layer of a first conduction-type formed on the first low refractive index layer; an active layer formed on the first semiconductor layer; a second semiconductor layer of a second conduction-type formed to be on and in contact with the active layer; a third semiconductor layer of the first conduction-type or the second conduction-type formed on the second semiconductor layer, above a formation region of the active layer; non-electroconductive fourth semiconductor layer and fifth semiconductor layer formed on the first semiconductor layer and being in contact with both lateral faces of a ridge pattern with the active layer and the second semiconductor layer; a second low refractive index layer having a lower refractive index than semiconductor, the second low refractive index layer being formed on the fourth semiconductor layer and being in contact with one of lateral faces of a ridge pattern with the third semiconductor layer; a third low refractive index layer having a lower refractive index than semiconductor, the third low refractive index layer being formed on the fifth semiconductor layer and being in contact with another of the lateral faces of the ridge pattern with the third semiconductor layer; a first electrode electrically connecting to the first semiconductor layer; and a second electrode electrically connecting to the third semiconductor layer, wherein a width of the third semiconductor layer, on the active layer side, in a direction perpendicular to a waveguide direction and parallel to a plane of the substrate is set to be smaller than a width of the active layer in the same direction. 
     There is provided a semiconductor optical device according to embodiments of the present invention, including: a first low refractive index layer having a lower refractive index than semiconductor, the first low refractive index layer being formed on a substrate; a first semiconductor layer of a first conduction-type formed on the first low refractive index layer; an active layer formed on the first semiconductor layer; a second semiconductor layer of a second conduction-type formed to be on and in contact with the active layer; a third semiconductor layer of the first conduction-type or the second conduction-type formed on the second semiconductor layer, above a formation region of the active layer; a second low refractive index layer and a third low refractive index layer having a lower refractive index than semiconductor, the second low refractive index layer and the third low refractive index layer being formed on the second semiconductor layer and being in contact with both lateral faces of a ridge pattern with the third semiconductor layer; a first electrode electrically connecting to the first semiconductor layer; and a second electrode electrically connecting to the third semiconductor layer, wherein a width of the third semiconductor layer, on the active layer side, in a direction perpendicular to a waveguide direction and parallel to a plane of the substrate is set to be smaller than a width of the active layer in the same direction. 
     Effects of Embodiments of the Invention 
     As described above, according to embodiments of the present invention, since there is employed a vertical pin structure in which a current is applied to the active layer in the up-down direction and layers having a lower refractive index than semiconductor are arranged above on and below on the active layer, high light confinement to the active layer can be realized without a leak path of current formed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a sectional view showing a configuration of a semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 A  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 B  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 C  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 D  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 E  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 F  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 G  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 H  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 I  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  2 J  is a sectional view showing a device structure in the middle of production for explaining a manufacturing method of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  3    is a sectional view showing a configuration of a semiconductor optical device according to Embodiment 2 of the present invention; 
         FIG.  4    is a sectional view showing another configuration of the semiconductor optical device according to Embodiment 1 of the present invention. 
         FIG.  5 A  is a distribution diagram showing the results of analyzing a distribution of holes in the active layer  104  by electromagnetic field distribution simulation. 
         FIG.  5 B  is a distribution diagram showing the results of analyzing a distribution of electrons in the active layer  104  by electromagnetic field distribution simulation. 
         FIG.  5 C  is a distribution diagram showing the results of analyzing a radiative recombination distribution in the active layer  104  by electromagnetic field distribution simulation. 
         FIG.  6 A  is a distribution diagram showing the results of analyzing a distribution of holes in the active layer  104  by electromagnetic field distribution simulation. 
         FIG.  6 B  is a distribution diagram showing the results of analyzing a radiative recombination distribution in the active layer  104  by electromagnetic field distribution simulation. 
         FIG.  7 A  is a characteristic diagram showing the results of examining a more desirable value of the width of the third semiconductor layer  106  of the semiconductor optical device according to Embodiment 1 described using  FIG.  1   , on the basis of the coefficient of light confinement to the active layer based on two-dimensional mode distribution calculation by a film mode matching method. 
         FIG.  7 B  is a characteristic diagram showing the results of examining a more desirable value of the width of the third semiconductor layer  106  of the semiconductor optical device according to Embodiment 1 described using  FIG.  1   , on the basis of the coefficient of light confinement to the active layer based on two-dimensional mode distribution calculation by a film mode matching method. 
         FIG.  7 C  is a characteristic diagram showing the results of examining a more desirable value of the width of the third semiconductor layer  106  of the semiconductor optical device according to Embodiment 2 described using  FIG.  3   , on the basis of the coefficient of light confinement to the active layer based on two-dimensional mode distribution calculation by a film mode matching method. 
         FIG.  7 D  is a characteristic diagram showing the results of examining a more desirable value of the width of the third semiconductor layer  106  of the semiconductor optical device according to Embodiment 2 described using  FIG.  3   , on the basis of the coefficient of light confinement to the third semiconductor layer based on two-dimensional mode distribution calculation by a film mode matching method. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Hereafter, semiconductor optical devices according to embodiments of the present invention are described. 
     Embodiment 1 
     Frist, a semiconductor optical device according to Embodiment 1 of the present invention is described with reference to  FIG.  1   . This semiconductor optical device firstly includes: a first low refractive index layer  102  having a lower refractive index than semiconductor, the first low refractive index layer  102  being formed on a substrate  101 ; a first first semiconductor layer  103  of a conduction-type formed on the first low refractive index layer  102 ; an active layer  104  formed on the first semiconductor layer  103 ; and a second semiconductor layer  105  of a second conduction-type formed to be on and in contact with the active layer  104 . Light is generated in the active layer  104 . The active layer  104  is elongated, for example, from this side to the back side of the view plane of  FIG.  1   . Moreover, the active layer  104  and the second semiconductor layer  105  are formed to have the same area in plan view, and the second semiconductor layer  105  is overlapped with the active layer  104  in plan view. 
     Moreover, this semiconductor optical device includes: a third semiconductor layer  106  formed on the second semiconductor layer  105 ; and non-electroconductive fourth semiconductor layer  107  and fifth semiconductor layer  108  formed on the first semiconductor layer  103  and being in contact with both lateral faces of a ridge pattern with the active layer  104  and the second semiconductor layer  105 . The third semiconductor layer  106  can be of the second conduction type. Otherwise, the third semiconductor layer  106  can be of the first conduction type. 
     Here, the third semiconductor layer  106  is arranged above a formation region of the active layer  104 . In other words, the bottom surface of the third semiconductor layer  106  is arranged in the formation region of the active layer  104 . Moreover, a width of the third semiconductor layer  106 , on the active layer  104  side, in a direction perpendicular to the waveguide direction and parallel to the plane of the substrate is set to be smaller than a width of the active layer  104  in the same direction. Notably, the direction perpendicular to the waveguide direction and parallel to the plane of the substrate is a direction sandwiched by the fourth semiconductor layer  107  and the fifth semiconductor layer  108 . Notably, as with the active layer  104 , the third semiconductor layer  106  is elongated from this side to the back side of the view plane of  FIG.  1   , for example. 
     Moreover, this semiconductor optical device includes: a second low refractive index layer  109  having a lower refractive index than semiconductor, the second low refractive index layer  109  being formed on the fourth semiconductor layer  107  and being in contact with one of lateral faces of a ridge pattern with the third semiconductor layer  106 ; and a third low refractive index layer  110  having a lower refractive index than semiconductor, the third low refractive index layer  110  being formed on fifth semiconductor layer  108  and being in contact with another of the lateral faces of the ridge pattern with the third semiconductor layer  106 . Moreover, there are included: a first electrode  111  electrically connecting to the first semiconductor layer  103 ; and a second electrode  112  electrically connecting to the third semiconductor layer  106 . For example, the second electrode  112  can be formed on the third semiconductor layer  106  via a contact layer  113 . 
     The substrate  101  can be composed, for example, of silicon. The first low refractive index layer  102  can be composed, for example, of a material having a lower refractive index than semiconductor that the active layer  104  is composed of, such as silicon oxide, silicon nitride, silicon carbide or diamond. The first low refractive index layer  102  functions as a lower cladding layer as viewed from the substrate  101  side. The first semiconductor layer  103  can be composed, for example, of n-type InP. In this case, the first conduction type is an n-type and the second conduction type is a p-type. 
     For example, the active layer  104  can be a multiple quantum well structure with a well layer and a barrier layer which are composed of InGaAlAs, InGaAs, InGaAsP or the like having different compositions. Moreover, the active layer  104  can also be composed of compound semiconductor such as bulk InGaAlAs, InGaAs or InGaAsP. The second semiconductor layer  105  can be composed, for example, of p-type InP. In this case, the second conduction type is the p-type. Moreover, the second semiconductor layer  105  can also be composed of semiconductor having a refractive index between those of the third semiconductor layer  106  and the active layer  104 . Configuring the refractive index differences as above can attain a separate confined heterostructure (SCH). 
     The third semiconductor layer  106  can be composed, for example, of p-type InP. Otherwise, the third semiconductor layer  106  can be composed, for example, of n-type InP. The fourth semiconductor layer  107  and the fifth semiconductor layer  108  can be composed, for example, of i-type InP (i-InP). Otherwise, the fourth semiconductor layer  107  and the fifth semiconductor layer  108  can also be composed of semi-insulating InP (SI—InP) which is given high resistance by doping with Fe or the like. 
     The second low refractive index layer  109  and the third low refractive index layer  110  can be composed, for example, of a material having a lower refractive index than the semiconductor that the active layer  104  is composed of, such as silicon oxide or silicon nitride. Otherwise, the second low refractive index layer  109  and the third low refractive index layer  110  can also be composed of a resin such as benzocyclobutene (BCB). The second low refractive index layer  109  and the third low refractive index layer  110  function as an upper cladding layer as viewed from the substrate  101 . The semiconductor optical device according to Embodiment 1 has a structure of an optical waveguide in which the first low refractive index layer  102  is the lower cladding layer, the active layer  104  is the core, and the second low refractive index layer  109  and the third low refractive index layer  110  are the upper cladding layer. 
     Now, a semiconductor layer, for example, of p-type InGaAsP or the like can also be arranged between the active layer  104  and the third semiconductor layer  106  (second low refractive index layer  109 , third low refractive index layer  110 ) in the thickness direction (stacking direction). As described for a manufacturing method mentioned later, this semiconductor layer can be caused to function as an etching stop layer for forming the third semiconductor layer  106 . 
     According to the aforementioned configuration, first, there is employed what we call a vertical pin structure in which, as viewed from the substrate  101  side, the right and the left of the active layer  104  are sandwiched by the non-electroconductive fourth semiconductor layer  107  and fifth semiconductor layer  108  and a current is to be applied to the active layer  104  in the up-down direction. Therefore, a leak path of current which becomes a problem on what we call a lateral pin structure is not formed, and the high light confinement to the active layer  104  can be realized. 
     Moreover, according to the aforementioned configuration, the width of the third semiconductor layer  106  which forms a structure of current injection to the active layer  104  is made smaller than that of the active layer  104  and the second low refractive index layer  109  and the third semiconductor layer  106  are formed at the periphery of the third semiconductor layer  106 , therefore, there is attained the state where layers of an insulating material are arranged above on and below on the active layer  104 , and the high light confinement to the active layer  104  can be realized. 
     Next, a manufacturing method of the semiconductor optical device according to Embodiment 1 is described with reference to  FIG.  2 A  to  FIG.  2 J . 
     First, as shown in  FIG.  2 A , the first low refractive index layer  102  is formed on the substrate  101 , and a lower InP layer  103   a  composed, for example, of n-type InP is formed on the first low refractive index layer  102 . For example, well-known wafer joining technology can form the lower InP layer  103   a  on the first low refractive index layer  102 . The lower InP layer  103   a  is a layer to be the first semiconductor layer  103 . 
     Next, as shown in  FIG.  2 B , a selective growth mask  131  composed, for example, of silicon oxide is formed at a predetermined place on the lower InP layer  103   a . The selective growth mask  131  can be formed, for example, by performing sputtering method or the like to deposit silicon oxide and to form a silicon oxide layer and processing (patterning) this silicon oxide layer with known lithography technology and etching technology. 
     Next, as shown in  FIG.  2 C , with selective growth using the selective growth mask  131 , an upper InP layer  103   b , a multiple quantum well layer  132  of InGaAlAs, and an InP cap layer  133  composed of p-type InP are formed on the exposed lower InP layer  103   a  sequentially in the same growth device. These can be formed through growth by a well-known organometallic vapor phase epitaxy method, molecular beam epitaxy method or the like. The lower InP layer  103   a  and the upper InP layer  103   b  constitute the first semiconductor layer  103 . 
     Next, as shown in  FIG.  2 D , the active layer  104  and the second semiconductor layer  105  are formed on the upper InP layer  103   b  by patterning the multiple quantum well layer  132  and the InP cap layer  133  with etching technology using a mask (not shown) of an inorganic material (for example, silicon oxide) formed by known lithography technology. The active layer  104  and the second semiconductor layer  105  are formed to have widths, in plan view, of 800 nm, for example. Moreover, the lower InP layer  103   a , the upper InP layer  103   b , the active layer  104  and the second semiconductor layer  105  are formed to have the total thickness of 250 nm. 
     Next, using the selective growth mask  131  and the aforementioned mask of the inorganic material having been used in the formation (patterning) of the active layer  104  and the second semiconductor layer  105  as a selective growth mask, non-doped InP or semi-insulating InP is regrown on the upper InP layer  103   b  that is exposed on the peripheries of the active layer  104  and the second semiconductor layer  105 . This growth of InP can be performed by a well-known organometallic vapor phase epitaxy method, molecular beam epitaxy method or the like. As shown in  FIG.  2 E , this results in formation of the fourth semiconductor layer  107  and the fifth semiconductor layer  108 . Next, as shown in  FIG.  2 F , an etching stop layer  114  composed of p-type InGaAsP is formed on the second semiconductor layer  105 , the fourth semiconductor layer  107 , and the fifth semiconductor layer  108 . 
     Next, as shown in  FIG.  2 G , a selective growth mask  134  which includes an opening  134   a  in a predetermined region is formed on the etching stop layer  114 . For example, the selective growth mask  134  is formed to be in the state of enclosing the periphery of the active layer  104  the shape of which in plan view is set to be rectangular. For example, the selective growth mask  134  is formed to be in the state where the active layer  104  is arranged in the center part of the opening  134   a  in plan view. Notably, the width of the selective growth mask  134  in plan view is in the same state, in any of the width direction and the elongation direction of the active layer  104  in plan view. 
     Next, through selective growth using the selective growth mask  134 , p-type InP is grown on the etching stop layer  114  that is exposed in the opening  134   a  to form an InP layer  135 . Moreover, p-type InGaAs is subsequently grown on the InP layer  135  to form an InGaAs layer  136 , for example, to have about 1000 nm of thickness. Herein, the width of the opening  134   a  in the planar direction of the substrate  101  can be about 1 μm to 50 μm, for example. The InGaAs layer  136  becomes the third semiconductor layer  106 . 
     Since as mentioned above, the InGaAs layer  136  is formed to have a smaller area in plan view by using the selective growth mask  134 , large residual stress can be more prevented from arising as compared with the case where this layer is formed on the whole region of the etching stop layer  114 . For the growth of the InP layer  135 , there is used the organometallic vapor phase epitaxy method, the molecular beam epitaxy method or the like, as with those of the semiconductor layers mentioned above. As is well known, in these growth methods, the process temperature becomes high temperature. 
     Here, in high temperature processing such as crystal growth as mentioned above, stress arises due to the difference in coefficient of thermal expansion between InP and the substrate  101  composed of silicon. 
     When the stress arises as mentioned above after the InP layer  135  grows to be thick, there can be a case where just an elastic stress relaxation cannot sufficiently compensate for it and a stress relaxation by the formation of cracks occurs in the InP layer  135 . Such formation of cracks in the InP layer  135  leads to a cause of damage to the active layer  104 , which is formed to be thin. 
     Against this, when the area where the InP layer  135  is formed is limited using the selective growth mask  134  as mentioned above, the elastic stress relaxation in the planar direction of the substrate  101  is possible, and cracks in the InP layer  135  in the region of the active layer  104  can be restrained from arising. Forming the InP layer  135  using the selective growth mask  134  as above makes it possible “to produce a thick InP-based layer structure on silicon through crystal growth”, which is typically difficult. 
     Next, by patterning the InP layer  135  and the InGaAs layer  136  with known lithography technology and etching technology, as shown in  FIG.  2 I , the third semiconductor layer  106  and the contact layer  113  are formed. After that, the selective growth mask  131  and the selective growth mask  134  are removed. 
     Next, as shown in  FIG.  2 J , the second low refractive index layer  109  and the third low refractive index layer  110  are formed. The second low refractive index layer  109  and the third low refractive index layer  110  can be formed in the targeted region, for example, by forming a coating film through coating with BCB having photosensitivity and patterning this coating film by known lithography technology. After the second low refractive index layer  109  and the third low refractive index layer  110  are formed as above, the first electrode  111  and the second electrode  112  are formed. These electrodes can be formed, for example, by a well-known lift-off method. 
     Embodiment 2 
     Next, a semiconductor optical device according to Embodiment 2 of the present invention is described with reference to  FIG.  3   . This semiconductor optical device firstly includes: the first low refractive index layer  102  formed on the substrate  101 ; and the first semiconductor layer  103  of a first conduction-type formed on the first low refractive index layer  102 . These are similar to those in Embodiment 1 mentioned above. 
     Moreover, this semiconductor optical device includes: an active layer  104   a  formed on the first semiconductor layer  103 ; and a second semiconductor layer  105   a  of a second conduction-type formed to be on and in contact with the active layer  104   a . While in Embodiment 2, the active layer  104   a  and the second semiconductor layer  105   a  is elongated, for example, from this side to the back side of the view plane of  FIG.  3   , they do not have a buried heterostructure (BH) as in Embodiment 1. In Embodiment 2, there is not formed a buried structure in which the active layer  104   a  and the second semiconductor layer  105   a  are sandwiched in the right-left direction of the view plane of  FIG.  3   . 
     Notably, the semiconductor optical device according to Embodiment 2 also includes: a third semiconductor layer  106  of the second conduction-type formed on the second semiconductor layer  105   a ; and second low refractive index layer  109   a  and third low refractive index layer  110   a  formed on the second semiconductor layer  105   a  and being in contact with both lateral faces of the ridge pattern with the third semiconductor layer  106 . As with the second low refractive index layer  109  and the third low refractive index layer  110 , the second low refractive index layer  109   a  and the third low refractive index layer  110   a  can be composed, for example, of a material having a lower refractive index than semiconductor that the active layer  104  is composed of, such as silicon oxide or silicon nitride. Also in Embodiment 2, the third semiconductor layer  106  is arranged above the formation region of the active layer  104   a . Moreover, the width of the third semiconductor layer  106 , on the active layer  104   a  side, in the direction perpendicular to the waveguide direction and parallel to the plane of the substrate is set to be smaller than the width of the active layer  104   a  in the same direction. 
     Moreover, there are included: the first electrode in electrically connecting to the first semiconductor layer  103 ; and the second electrode  112  electrically connecting to the third semiconductor layer  106 . Also in Embodiment 2, the second electrode  112  can be formed, for example, on the third semiconductor layer  106  via the contact layer  113 . 
     In Embodiment 2, using (utilizing) the refractive index differences of the third semiconductor layer  106  from the second low refractive index layer  109   a  and the third low refractive index layer  110   a , the width of the surface (bottom surface) of the third semiconductor layer  106 , on the active layer  104   a  side, defines the region of light confinement in the right-left direction of the active layer  104   a  and configures the structure of an optical waveguide. 
     Also in Embodiment 2, first, there is employed what we call the vertical pin structure in which a current is to be applied to the active layer  104   a  in the up-down direction as viewed from the substrate  101  side. Therefore, the leak path of current which becomes a problem on what we call the lateral pin structure is not formed, and the high light confinement to the active layer  104   a  can be realized. Moreover, since also with the aforementioned configuration, layers of an insulating material are arranged above on and below on the active layer  104   a , the high light confinement to the active layer  104   a  can be realized. 
     Notably, also in Embodiment 2, a semiconductor layer, for example, of p-type InGaAsP or the like can also be arranged between the active layer  104   a  and the third semiconductor layer  106 . As mentioned above, this semiconductor layer can be caused to function as the etching stop layer for forming the third semiconductor layer  106 . Moreover, also in Embodiment 2, the second semiconductor layer  105   a  can be composed of semiconductor having a refractive index between those of the third semiconductor layer  106  and the active layer  104   a.    
     Now, as shown in  FIG.  4   , in the semiconductor optical device (refer to  FIG.  2 J ) described for Embodiment 1 mentioned above, there can also be employed a configuration including a core  115  buried in the first low refractive index layer  102  in the region below the active layer  104  (third semiconductor layer  106 ). The core  115  can be composed, for example, of Si or hydrogenated amorphous silicon. As with the active layer  104 , the core  115  is elongated from this side to the back side of the view plane of  FIG.  4   . For example, the core  115  is separate from the active layer  104  in a range where they can be optically coupled to each other. For example, light generated in the active layer  104  optically couples to the core  115  and can be taken out from the optical waveguide of the core  115 . 
     For example, in the step of the manufacturing method described using  FIG.  2 A , the core  115  is beforehand formed in the first low refractive index layer  102  by wafer joining technology before the lower InP layer  103   a  is formed on the first low refractive index layer  102 . For example, after the first low refractive index layer  102  that the core  115  is buried in is formed on the substrate  101  and its surface is flattened by chemical mechanical polishing (CMP), the lower InP layer  103   a  can be formed. Otherwise, there can also be employed a configuration including a diffraction grating buried in the first low refractive index layer  102  in the region below the active layer  104  (third semiconductor layer  106 ). Moreover, the diffraction grating can also be formed in the core  115 . These configurations also apply to the semiconductor optical device according to Embodiment 2 described using  FIG.  3   . 
     Next, there are described the results of examination on the effects of embodiments of the present invention through calculations. The semiconductor optical devices according to the embodiments described above are characterized in that in order to confine light to the active layer  104 , the width of the third semiconductor layer  106  in the shape of the cross section perpendicular to the waveguide direction is smaller than the width of the active layer  104 . It was herein analyzed, by electromagnetic field distribution simulation, how carriers injected into the active layer  104  diffuse and distribute in the active layer  104 . 
     Hereafter, the results of the analysis by the simulation are described. With the configuration described using  FIG.  1   , the simulation employed the third semiconductor layer  106  the width of which was 400 nm, and the active layer  104  that had a multiple quantum well structure in which six sets of quantum well layers and barrier layers were stacked. Moreover, the total thickness of the first semiconductor layer  103 , the active layer  104 , and the second semiconductor layer  105  was 250 nm. Moreover, the width of each of the fourth semiconductor layer  107  and the fifth semiconductor layer  108  was 1 μm. Moreover, 1.6 V of voltage was applied between the first electrode  111  and the second electrode  112 . 
     Under the aforementioned conditions, a hole density distribution was obtained as shown in  FIG.  5 A , therein, no significant distribution is recognized in the right-left direction of the active layer  104 , and a symmetric distribution is shown in the up-down direction thereof. The facts above reveal that in the semiconductor optical device according to the embodiment, holes can be appropriately injected into the active layer  104 . Notably, it is clear from  FIG.  5 A  that no holes (carriers) or the like distribute in the barrier layers (white portions). The same holds true for  FIG.  5 B ,  FIG.  5 C ,  FIG.  6 A  and  FIG.  6 B  illustrated below. 
     Moreover, an electron density distribution was obtained as shown in  FIG.  5 B , therein, as with the hole density distribution, no significant distribution is recognized in the right-left direction of the active layer  104 , and a symmetric distribution is shown in the up-down direction thereof. The facts above reveal that in the semiconductor optical device according to the embodiment, electrons can be appropriately injected into the active layer  104 . 
     Moreover,  FIG.  5 C  shows a radiative recombination distribution. Reflecting the hole density distribution and the electron density distribution, there is shown a radiative recombination distribution which is uniform in the right-left direction of the active layer  104  and symmetric in the up-down direction thereof. These results reveal that the semiconductor optical device according to the embodiment can be applied to light-emitting devices such as a laser. 
     Next, for the case using the etching stop layer  114  composed of p-type InGaAsP, the results of analyzing a carrier distribution in the active layer  104  by the electromagnetic field distribution simulation as with the above are presented in  FIG.  6 A  and  FIG.  6 B . In this simulation, the width of the third semiconductor layer  106  was 200 nm. Such a condition to make the width of the third semiconductor layer  106  smaller is in a direction of more restraining in view of diffusion of holes in the right-left direction. 
     As shown in  FIG.  6 A , also in the case using the etching stop layer  114 , a hole density distribution uniform in the right-left direction of the active layer  104  and symmetric in the up-down direction thereof is obtained. Moreover, as shown in  FIG.  6 B , also in the case using the etching stop layer  114 , a radiative recombination distribution uniform in the right-left direction of the active layer  104  and symmetric in the up-down direction thereof is obtained. Also in such a case where the etching stop layer  114  or the like is used and the width of the third semiconductor layer  106  is made small to be down to 200 nm, it is clear that the semiconductor optical device according to the embodiment can be applied to light-emitting devices such as a laser. 
     Next, for a more desirable value of the width of the third semiconductor layer  106 , the results of examination with two-dimensional mode distribution calculations by a film mode matching method are described. There were employed below the third semiconductor layer  106  the width of which was a variable “x”, and the active layer  104  that had a multiple quantum well structure in which six sets of quantum well layers and barrier layers were stacked. Moreover, the thickness of the active layer  104  was 100 nm. Moreover, the total thickness (slab thickness) of the first semiconductor layer  103 , the active layer  104 , and the second semiconductor layer  105  was 250 nm or 350 nm. 
     First, the calculation results of the semiconductor optical device according to Embodiment 1 described using  FIG.  1    are presented in  FIG.  7 A  and  FIG.  7 B . In this example, the width of the active layer  104  was 800 nm. At the width “x” of the third semiconductor layer  106  being 0 nm, the coefficient of light confinement to the active layer  104  is about 40%. This corresponds to the same state as that of the conventional lateral pin structure. Being compared with this, as the width of the third semiconductor layer  106  is being made larger, the structure of light confinement to the active layer  104  in the up-down direction is disappearing, the coefficient of light confinement to the active layer  104  is largely lowering, and the coefficient of light confinement to the third semiconductor layer  103  is rising. This corresponding to the similar state to that of the conventional vertical pin structure, which causes deterioration of characteristics, for example, in the case of driving as a laser, such as a rise of an oscillation threshold and a fall of modulation efficiency due to the deterioration of light confinement to the active layer, and a rise of an oscillation threshold and a fall of optical output due to absorption at the valence band originated from the rise of the coefficient of light confinement to the third semiconductor layer. 
     When the width “x” of the third semiconductor layer  106  is not more than a certain value, it is clear that the coefficient of light confinement to the active layer  104  is not impaired and the coefficient of light confinement to the third semiconductor layer is still low. Referring to  FIG.  7 A , it is clear that, for example, when setting it as a reference that the coefficient of light confinement to the active layer  104  is not less than 80% as compared with that at the width “x” of the third semiconductor layer  106  being 0 nm, the width “x” of the third semiconductor layer  106  is still preferably not more than about 400 nm of width at 250 nm of slab thickness, and not more than about 500 nm of width in the case of 350 nm of slab thickness. Likewise, it is also clear that, for example, when setting it as a reference that the coefficient of light confinement to the third semiconductor layer  106  is not more than 20%, the width “x” of the third semiconductor layer  106  is still preferably not more than about 400 nm of width at 250 nm of slab thickness, and not more than about 500 nm of width in the case of 350 nm of slab thickness. 
     Next, the calculation results of the semiconductor optical device according to Embodiment 2 described using  FIG.  3    are presented in  FIG.  7 C  and  FIG.  7 D . Also with this structure, a still preferable width “x” of the third semiconductor layer  106  is not very different from that in the aforementioned case, and is 400 nm in the case of 250 nm of slab thickness, and not more than about 500 nm in the case of 350 nm of slab thickness. 
     The aforementioned results reveal that when the width of the third semiconductor layer  106  is set to be small to a certain extent or less in any of the semiconductor optical devices according to Embodiments 1 and 2, there can be obtained light confinement to the active layer  104   a  to the similar extent to that with the conventional lateral pin structure. For example, it can be considered that in the case of 250 nm of slab thickness, the width of the third semiconductor layer  106  is set to be not more than about 400 nm in order to obtain more sufficient light confinement. 
     Now, while in the aforementioned embodiments, there have been exemplarily illustrated the cases where the third semiconductor layer  106  is mainly of the second conduction type (p-type) and the semiconductor optical device is a light-emitting device such as a laser, the third semiconductor layer  106  can be of the first conduction type (n-type) and be an optical modulator. In this case, it is used under reverse bias application. 
     As described above, according to embodiments of the present invention, since there is employed a vertical pin structure in which a current is applied to the active layer in the up-down direction and layers of an insulating material are arranged above on and below on the active layer, high light confinement to the active layer can be realized without a leak path of current formed. 
     Notably, the present invention is not limited to the above-described embodiments, and it is obvious that those skilled in the art can carry out many variations and combinations thereof without departing from the technical concept of the present invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               101  Substrate 
               102  First low refractive index layer 
               103  First semiconductor layer 
               104  Active layer 
               105  Second semiconductor layer 
               106  Third semiconductor layer 
               107  Fourth semiconductor layer 
               108  Fifth semiconductor layer 
               109  Second low refractive index layer 
               110  Third low refractive index layer 
               111  First electrode 
               112  Second electrode 
               113  Contact layer.