Abstract:
In an edge emitting laser having a window region with a ridge-waveguide structure, particularly, in a short cavity type of a laser operated with a low current, there has been a problem of its operating current being increased due to current leakage of the window portion. To solve this problem, in the window region, between an n-type substrate and a p-type cladding layer, a semi-insulating semiconductor layer into which Ru is doped is inserted. Alternatively, a stacked structure of a Ru-doped layer and a Fe-doped layer is introduced.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese application JP 2007-301662, filed on Nov. 21, 2007, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to semiconductor laser diodes. More particularly, it relates to semiconductor laser with a ridge-waveguide structure and an electro-absorption modulator integrated DFB laser carrying the same. 
         [0004]    2. Description of the Related Art 
         [0005]    Recently, the demands for greater transmission capacities and increase in transmission speed are rapidly growing with the explosive increase in the Internet users, and it is considered that optical communications will play an important role in the future, too. Semiconductor lasers are widely used as light sources for optical communication systems. With the spread of optical communications, not only increase in modulation speed but also reduction in power consumption and reduced costs are strongly required. Various type of semiconductor lasers having different emission wavelengths are used for suitable applications, especially for suitable transmission distances. For short-reach applications whose transmission distance is 10 km or less, a directly modulated semiconductor laser with 1.3 μm wavelength band is mainly used. 
         [0006]    In the case of the directly modulated lasers, because a optical module is realizable with a simple constitution, its power consumption is small. Since the number of parts consisting of the module can be reduced, reduction in cost is also possible. The transmission speed of such optical modules in practical use is now becoming 10 Gb/s. As directly modulated semiconductor lasers, there are a vertical-cavity surface-emitting laser (VCSEL) and an edge-emitting laser (EEL) Owing to a cavity length of several μm, VCSEL operates with an operation current of about several to ten mA, and, thus, its power consumption is small. Further, a laser beam is emitted at vertical direction to a substrate. So, the sorting of laser chip is possible in the state of wafers before cleaving process. Thus, VCSEL is also good for cost reduction. However, when using a VCSEL with 1.3 μm wavelength band which is suitable for a standard single-mode fiber, in case of the transmission distance of several km, optical output power of several mW required for transmission cannot easily be obtained. This is due to small aperture region of about 5 μm in VCSEL to emit a single-mode laser beam. 
         [0007]    The cavity length of the conventional EEL is limited to about 200 μm. Therefore, for the high-speed operation of 10 or more Gbits/s, typical drive current is about 60 mA or more. As a result, in case of conventional EEL, it is difficult to further reduction in the power consumption. Therefore, as a new directly modulated type light source taking its place, a short cavity type of a laser described in Patent document 1 (JP-A No. 2007-5594) is proposed. In order to achieve both the low-current operation, which is the key to a low power consumption, and the optical output of several mW necessary for transmission distance of several km, the length of an active layer region is shortened. Further, the cavity length of a laser is set to a value within a range of from 10 to 100 μm which is an intermediate value of those of the conventional VCSEL and EEL. Thus, assuming that the upper limit of a drive current is set to about 80 mA, it is shown by calculation that a maximum value of the relaxation oscillation frequency is obtained under such cavity-length conditions. At the same time, there are also proposed an cost-effective structure which has slant reflecting mirror for converting the direction of a laser beam to be that of the surface emitting type and which has a lens for focusing the laser beam. 
         [0008]      FIG. 1  shows a bird&#39;s eye view of a short cavity type of a laser. The laser structure is of a distributed Bragg reflector (DBR) type.  FIG. 2  shows a cross-sectional structure of the short cavity type of a laser. Numeral  101  and  201  are lower electrodes;  102  and  202  are n-InP substrates;  103  and  203  are HR coating films;  104  and  204  are active layers;  105  and  205  are p-InP cladding layers;  106  and  206  are contact layers;  107  and  207  are upper electrodes;  108  and  208  are diffraction gratings;  109  and  209  are InGaAsP layers of a DBR region;  110  and  210  are slant reflectors; and  111  and  211  are back-surface lenses. Here, lengths of the active layers  104  and  204  are set to values within a range of from 10 μm to 100 μm. A distributed-feedback (DFB) type may also be applied to the present laser. In that case, the diffraction grating is formed above or under the active layer, and InP layers are often used instead of the InGaAsP layers  109  and  209  of the DBR region. This InP layer may also be formed at the same time that the p-InP cladding layer is formed. In such a case, pn junction of InP is may formed. At this time, as described in Patent document 2 (JP-A No. 2004-235182), it is preferable to form a reflecting mirror composed of semiconductor layers having two kinds of different refractive indices under the active layer. 
         [0009]    With respect to structures of semiconductor laser diodes, there are roughly two kinds, that is, a ridge waveguide (RWG) structure and a buried-hetero (BH) structure. This is the same for the previously described short cavity type of a laser.  FIGS. 3A and 3B  show a cross-sectional view perpendicular to the mesa stripe direction and a cross-sectional view parallel to the mesa stripe direction of the RWG structure and the BH structure of the DBR-type lasers, respectively. In  FIG. 3A , numeral  301  is an n-InP substrate;  302  is an active layer;  303  is a p-InP cladding layer;  304  is a contact layer;  305  is a diffraction grating;  306  is an InGaAsP layer in a DBR region;  307  is a slant reflector; and  308  is a back-surface lens. Further, in  FIG. 3B , numeral  309  is an n-type InP substrate;  310  is an active layer;  311  is a p-InP cladding layer;  312  is a contact layer;  313  is a diffraction grating;  314  is an InGaAsP layer in the DBR region;  315  is a slant reflector;  316  is a semi-insulating InP buried layer; and  317  is a back-surface lens In this case, for the simplicity of the explanation, electrodes and HR coating films are not illustrated. Also, a region including the portion of the slant reflector at the edge of the active layer is generally called a window region. In the conventional EEL, a window region is sometimes formed to suppress the interference between the optical feedback reflected at a cleaved facet and a laser beam. 
         [0010]    In the RWG structure shown in  FIG. 3A , when forming a mesa structure having a width of several μm by etching the upper cladding layer  305 , etching is stopped above the active layer  302 . Since the active layer portion is not etched, excessive damage to the active layer is not caused, which is advantageous in terms of highly reliable operation of the laser. On the other hand, in the BH structure shown in  FIG. 3B , when forming a mesa, etching is performed deeply enough to a portion below the active layer  310 . Further, a high-insulating semiconductor layer  316  is again buried in both sides of the mesa including the active layer. Therefore, sides of the active layer may be damaged during mesa etching, and its quality may be degraded. Particularly, in the active layers including an InGaAlAs material for 1.3 μm wavelength band containing high-content Al atoms, a chemically robust oxides is formed at the side of the active layer after the mesa etching process. As a result, a better buried hetero epitaxial growth is disturbed. Therefore, in order to overcome this difficulty, a special treatment for the side of the active layer is often necessary just before a crystal growth, and, thus, it is not easy to realize highly reliable operation of the laser. 
         [0011]    Further, as described earlier, when integrating slant reflectors in the window region, in order to increase its reflective efficiency of the laser beam and allow the laser beams to focus on the lens effectively, a precise control of the etching angle of the slant reflector is indispensable. Since the slant reflectors are formed by dry etching or wet etching, when considering the controllability of the etching angle, it is preferred that a surface of the window region before etching should be flat. When comparing the RWG structure with the BH structure from this point of view, the surface of the window region of RWG structure in which the p-InP cladding layer  303  is re-grown on a surface where the active layer  302  and the InGaAsP layer  306  are smoothly joined in lateral direction, is more flatter than that of the BH structure in which the semi-insulating InP buried layer  316  is re-grown on a surface with a large thickness difference of more than 2 μm formed by mesa etching. It is preferred that the surface is flat in terms of manufacturing an laser with a high yield in the manufacturing process to be followed. Therefore, for the short cavity type of a laser, when comparing the RWG structure with the BH structure in terms of reliable operation and surface flatness, it is understood that the RWG structure is more advantageous than the BH structure. 
       SUMMARY OF THE INVENTION 
       [0012]    Now, problems of the EEL having a window region with a RWG structure, particularly, of the short cavity type of a laser will be described. In order to operate a laser, it is necessary to apply a voltage of a forward direction between a p-InP cladding layer and an n-InP substrate through an electrode, and to inject an electric current into the active layer  302 . The applied voltage usually spreads over an area larger than an electrode portion by several μm. Therefore, in the RWG structure of the DBR-type laser in  FIG. 3A , in the window region, a forward voltage is unintentionally applied to the portion near the active layer of the InGaAsP layer  306 . Since the intentional doping is not given to the InGaAsP layer  306 , as a typical value, the background carrier concentration of the InGaAsP layer is about 5×10 14  cm −3  in the n-type, and, thus, the resistivity of the layer is about 1 Ωcm. Therefore, the some current flows through a part of the InGaAsP layer when the laser is operated. This leak current, which is not injected into the active layer, degrades laser characteristics. Moreover, in the DFB-type laser described earlier, the window region is the pn junction of InP, and the leak current becomes larger. On the other hand, in the BH structure of the FIG.  3 B(b), the semi-insulating InP buried layer  316  (whose resistivity is more than 5×10 7  ωcm) is formed in the window region with a thickness of about several μm. Therefore, the leak current is suppressed. Moreover, in the RWG structure, a surface current flows in the pn junction on the surface of the slant reflector. In particular, this current tent to increase when certain substances adheres to the surface or when the surface is covered with an insulating film. This also increases a leak current and degrades laser characteristics. As the cavity length becomes shorter, a threshold current becomes lower and the proportion of the leak current component to an operating current becomes higher. Therefore, the leak current serious influences laser characteristics, especially for the short cavity type of a laser. As a result, it disturbs the operation at a low drive current. Further, additional reason for the increase of leak current is due to the increase of the resistance of laser with decrease of the cavity length. Patent document 3 (JP-A No. 2003-110195) is reported on the problem of the leak current of the conventional EEL which has a window region. In order to suppress the leak current in the window region, the Fe doped semi-insulating layer with the same thickness as that of the active layer is inserted. InP, InGaAsP and others are disclosed as Fe-doped semi-insulating layers. By doping Fe using a well-known technology, the resistivity of the InP single layer to be inserted, for example, can be increased to about 5×10 7  Ωcm. However, when stacking with a p-doping layer and Fe-doped semi-insulating layer is actually considered, the situation is quite different. It is known that Fe causes inter diffusion with Zn which is generally used as p-type dopant in the metal-organic vapor phase epitaxy (MOVPE) method. For example, in Applied Physics Letters, vol. 67, 1995, pp. 479-481,  FIG. 2(   b ), the result of the secondary ion mass spectrometry when a Zn doped InP layer is stacked on the Fe doped InP layer is shown. According to  FIG. 2(   b ), it is seen that Zn atoms are diffused into the Fe doped InP layer, and Fe atoms are diffused into the Zn doped layer. At the interface between the Fe doped InP layer and the Zn doped InP layer, the Fe atoms are depleted at around 300 nm near the Fe doped layer side. Further, Zn atoms(with resistivity of about 1 Ωcm) of about 5×10 16  cm −3  is diffused there. Therefore, it is conceivable that considerable decrease in the resistivity at the stack interface takes place as a result. Generally, since the thickness of the active layer  302  of a laser is 300 nm or less, the thickness of the Fe-doping layer to be inserted is almost the same. Moreover, even if the thickness of the Fe-doping layer is set to be thicker than that of the active layer, the thickness near the junction with the Fe-doping layer and the active layer  302  does not change. As a result, even if the Fe doping is applied to the InGaAsP layer or the InP layer of the window region by the conventional technology, it is not possible to suppress the leak current completely. 
         [0013]      FIG. 4  shows the estimation of the leak current as a function of the resistivity of the semiconductor layer to be inserted in the window region. In  FIG. 4 , the thickness of the layer to be inserted in the window region is set to be 0.3 μm, which is almost the same as that of the active layer. The leak area is assumed to be 2 μm in width and extends 1 μm from the edge of the active layer. Also, the applied voltage is also assumed to be 1V. When the resistivity is 1·cm which is equivalent to the resistivity of the undoped layer described earlier or resistivity of the Fe-doping layer after the Zn diffusion, the leak current was estimated to be about 0.7 mA. Compared to the threshold current of about 10 mA of the conventional laser whose cavity length is 200 μm, this value is not so large. However, when compared with the threshold of several mA for the short cavity type of a laser referred to in the present invention, 0.7 mA is a high value exceeding by 10% of its threshold current. Although the result shown in  FIG. 4  is simply one of the various operating conditions of the laser, the general tendency is almost the same. Therefore, it can be said that it is difficult to completely suppress the leak current in the window region by using the conventional technology. 
         [0014]    In view of the above, the present invention is to solve the problem of suppressing the leak current in the window region, which has considerable influence on EEL with a RWG structure having a window region and, in particular, on a short cavity type of a laser with a low operating current. 
         [0015]    In order to solve the above problem, the present inventors devised following two novel laser structures. A first structure of the present invention is an RWG-type semiconductor laser which includes, on a semiconductor substrate, an active layer for generating light, upper and lower cladding layers for confining the generated light, and a cavity structure in a direction parallel to the substrate, and has a mesa structure formed above the active layer. The semiconductor laser structure has a window region composed of a semiconductor layer whose band gap is larger than the energy of the laser beam. The window region is composed of a stacked structure of an n-type semiconductor layer, a semi-insulating semiconductor layer, and a p-type semiconductor layer. In the semiconductor layer structure, a layer into which a material selected from the group of Ru, Os, Rh, and Ti is doped is used for part of or whole of the semi-insulating semiconductor layer.  FIG. 5  shows its specific example. The laser is the DBR type. In addition, the semiconductor material used for the semiconductor laser is made of combinations of elements selected from the group of In, Ga, Al, B, Tl, Bi, As, P, Sb, and N. In  FIG. 5 , numeral  501  is a lower electrode;  502  is an n-InP substrate,  503  is an HR coating film;  504  is an active layer;  505  is a p-InP cladding layer;  506  is a contact layer;  507  is an upper electrode;  508  is a diffraction grating;  509  is a Ru-doped InGaAsP layer;  510  is a slant reflector; and  511  is aback-surface lens. According to the proceedings of the international conference on indium phosphorus and related materials, 1998, TuA2-p357, A. Dadgar et al., the result of the secondary ion mass spectrometry when a Zn-doped InP layer is stacked on an Ru-doped InP layer is shown. According to the report, when Ru is used for dopant, an InP film can be formed into a high resistivity like Fe, however it is shown that, differing from Fe, the interdiffusion with Zn does not take place. As a result, doping concentration of Ru can also be made high compared with Fe. Therefore, the resistivity of a Ru dopded semiconductor layer can also be raised to about 1×10 5  ωcm level, which is higher than that of Fe by one order magnitude, and the resistivity does not fall by stacking with the Zn doped p-InP layer. Therefore, the leak current at the edge of the active layer  504  and the surface of the slant reflector can be suppressed by giving Ru doping to the InP layer of the window region and the InGaAsP layer sandwiched by the n-InP substrate and the p-InP cladding layer. As a result, it becomes possible to manufacture the short cavity type of laser with the RWG-waveguide structure which operates at a low current. Besides Ru, as doping atoms, which have no interdiffusion with Zn and are capable of allowing the InP etc. to be higher resistors, there are Os, Rh, Ti, etc. available, and it is possible to obtain similar effects by using them. 
         [0016]    A second structure of the present invention is an RWG-type semiconductor laser which includes, on a semiconductor substrate, an active layer which generates light, a cladding layer adapted to confine the generated light, and a cavity structure in a direction parallel to the substrate, and a mesa structure is formed above the active layer. The second laser structure has a window region composed of a semiconductor layer with a band gap larger than the energy of a laser beam. The window region is composed of a stacked structure of an n-type semiconductor layer, a semi-insulating semiconductor layer, and a p-type semiconductor layer. Part of or whole of the semi-insulating semiconductor layer has a stacked structure of a layer into which a material selected from the group of Ru, Os, Rh, and Ti is doped and a layer into which Fe is doped. The layer into which Fe is doped is stacked continuously with the p-type semiconductor layer into which Zn is doped.  FIG. 6A  shows its specific example. The element used was of the DFB type. In  FIG. 6A , numeral  601  is a lower electrode;  602  is an n-InP substrate;  603  is an HR coating film;  604  is an active layer;  605  is a diffraction grating;  606  is a contact layer;  607  is an upper electrode;  608  is a p-InP cladding layer;  609  is an Fe-doped InP layer;  610  is an Ru-doped InP layer;  611  is a slant reflector;  612  is an n-InP buffer layer;  613  is a multilayer reflecting mirror; and  614  is a back-surface lens. 
         [0017]    This is a structure for further enhancing the suppression effect of the leak current in the first element structure described earlier. Specifically, the InP layer of the window region sandwiched by the n-InP substrate and the p-InP cladding layer is formed as a stacked structure of the Ru-doping layer and the Fe-doping layer. In this regard, the Fe-doped InP is positioned next to the p-InP cladding layer. As stated earlier, an Fe atom has a property of having interdiffusion with Zn which is p-type dopant. Conversely interpreted, the Fe atoms are diffused in the Zn-doping layer and are effective in reducing effectual p-type doping concentration. With the structure shown in  FIG. 6A , in the A region, Fe is spread from an Fe-doping layer into the p-InP layer, and Zn is conversely spread into the Fe-doping layer from the Zn-doping layer. Since the effectual p-type concentration of the p-type layer falls, its resistivity increases. This brings about the effect of reducing the leak current. Therefore, the leak current suppression effect can further be raised as compared with the first laser structure by a synergistic effect with the high resistive lower layer into which Ru is doped. 
         [0018]    According to the result shown in  FIG. 4 , by applying the layer (resistivity:about 1×10 9  Ωcm) into which Ru is doped, it is seen that the leak current can be reduced to a very small value (in this result, about 1×10 −9  mA). 
         [0019]    By using the structure of the present invention described above, it is possible to suppress the leak current in the EEL having a conventional window region with a RWG structure, particularly, in the short cavity type of a laser, and to realize the operation at a low current. 
         [0020]    Further, the present invention is similarly applicable to other optical devices (for example, an electro-absorption modulator integrated DFB laser) having a window region. In a modulator, unlike a laser, the reverse bias voltage is applied. In this case, by applying the invention, generated photo carriers can effectively be guided to an electrode, which has an effect of suppressing the pile up of the photo carries. 
         [0021]    In a EEL with a RWG structure, particularly, in a short cavity type of a laser, in order to suppress the increase in the operating current caused by the current leak at the window region, a semiconductor layer of a semi-insulation property into which Ru is doped is inserted between the n-type substrate and the p-type cladding layer. Or the stacked structure of a layer into which Ru is doped and an Fe-doping layer is introduced Accordingly, the leak current can be suppressed and the low-current operation as designed can be realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a bird&#39;s eye view of a conventional short cavity laser; 
           [0023]      FIG. 2  shows a cross-sectional view of the conventional short cavity laser; 
           [0024]      FIG. 3A  shows an RWG structure of the conventional short cavity laser; 
           [0025]      FIG. 3B  shows an BH structure of the conventional short cavity laser; 
           [0026]      FIG. 4  shows the relationship between the resistivity of a film to be inserted in a window region and a leak current; 
           [0027]      FIG. 5  shows a cross-sectional view of an InGaAlAs short cavity laser of the present invention; 
           [0028]      FIG. 6A  shows a cross-sectional view of the InGaAlAs short cavity laser of the present invention; 
           [0029]      FIG. 6B  shows a cross-sectional view of the InGaAlAs short cavity laser of the present invention; 
           [0030]      FIG. 7  shows a cross-sectional view of a GaInNAs short cavity laser of the present invention; and 
           [0031]      FIG. 8  shows a cross-sectional view of an electro-absorption modulator integrated DFB laser of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    Examples of the present invention will be explained with reference to  FIG. 5 ,  FIGS. 6A and 6B , and  FIG. 7  and  FIG. 8 . 
       EXAMPLE 1 
       [0033]    Example 1 is a case where an Ru-doping layer alone is used to suppress a leak current in the window region. The laser is of a DBR type. The MOVPE method was used as a growth method. As source materials of group-III elements, triethyl gallium (TEG), trimethyl indium (TMI), and trimethyl aluminum (TMA) were used. As source materials of group-V elements, arsine (AsH 3 ) and the phosphine (PH 3 ) were used. Moreover, disilane (Si 2 H 6 ) was used as n-type dopant, and dimethyl zinc (DMZ) was used as p-type dopant. Furthermore, as an Ru source material, bis (η 5-2,4-dimethylepentadienyle ruthenium(II)) was used. However, materials are not limited to the above as long as they contain Ru. In addition, growth methods are not limited to the MOVPE method. For example, a molecular beam epitaxy (MBE) method, a chemical beam epitaxy (CBE) method, and a metal-organic molecular beam epitaxy (MOMBE) method, etc. can be used. 
         [0034]      FIG. 5  is a cross-sectional view of the laser. First, a laser part multiple-quantum-well (MQW) active layer  504  made of InGaAlAs is grown on an n-InP substrate  502 . Next, a mask of a desired shape was formed. After removing an unnecessary MQW region by etching, an InGaAsP layer  509  into which Ru was doped was formed by butt-joint regrowth. After the mask was removed, a necessary portion was covered again with a mask and a diffraction grating  508  was formed. After removing the mask, a p-InP cladding layer  505  and a contact layer  506  were grown on entire region, and the crystal growth process was ended. Next, a necessary portion was protected with a mask and a slant reflector  510  was formed by wet etching. Then, amesa was formed by using a mesa mask on a stripe. Furthermore, a back-surface lens  511  was formed by etching after polishing the substrate. After that, a passivation film is formed by using an ordinary manufacturing method, planarization is performed by using polyimide, and upper and lower electrodes  507  and  501  were formed to complete the laser. After cleavage, the HR coat  503  was applied to one cleavage end side of the manufactured laser. 
         [0035]    The 200 μm long laser having a MQW active layer part of 75 μm length was operated with 10 mA current drive at 10 GHz at 100° C. The laser beam was reflected by the slant reflector and was focused by the back-surface lens, and emitted from the back-surface side. The laser characteristics did not deteriorate even during the long term operation, showing the high reliability. Moreover, the manufacturing yield of the laser was also high. 
       EXAMPLE 2 
       [0036]    Example 2 is a case of using the stacked structure of an Ru-doping layer and an Fe-doping layer for suppressing a leak current in a window region. The element is of a DFB type. Although the MOVPE method was used here, growth methods are not limited to it. Other techniques may be used as long as similar effects are obtainable. In addition to the case of Example 1, ferrocene was used as dopant of Fe for the materials used. 
         [0037]      FIG. 6A  is a cross-sectional view of the laser. First, the multilayer reflecting mirror  613  composed of n-InGaAlAs/InP was formed on the n-InP substrate  602 . After forming the n-InP buffer layer  612 , the laser part MQW active layer  604  made of InGaAlAs was grown. Then, a mask of a desired shape was formed. After removing an unnecessary MQW region by etching, the Ru-doped InP layer  610  and Fe-doped InP layer  609  were formed by butt-joint regrowth. After removing the mask, a necessary portion was covered again with a mask and the diffraction grating  605  was formed. After removing the mask, the p-InP cladding layer  608  and contact layer  606  were grown on entire region, and the crystal growth process was ended. Next, a necessary portion was protected with a mask and the slant reflector  611  was formed by dry etching. Then, amesa was formed by using a mesa mask on a stripe. Furthermore, the back-surface lens  614  was formed by etching after polishing the substrate. Then, according to an ordinary manufacturing method, a passivation film was formed, planarization was performed by using polyimide, and upper and lower electrodes  607  and  601  were formed to complete the element. After cleavage, the HR coat  603  was applied to the back surface of the manufactured laser. 
         [0038]    The 200 μm long laser having a MQW active layer part of 50 μm length was operated with 7 mA current drive at 10 GHz at 100° C. The laser beam was reflected off the slant reflector, and was focused by the back-surface lens, and emitted from the back-surface side. The laser characteristics did not deteriorate even during long term operation, showing high reliability. Also, the manufacturing yield of the laser was high. As described in Example 1, even if the Ru-doping layer alone is used, it is possible to obtain the effects of the present invention. In that case, the structure will be as shown in  FIG. 6B . 
       EXAMPLE 3 
       [0039]    Example 3 is, like Example 2, a case where the Ru-doping layer alone was used to suppress a leak current in the window region. The element was of a DBR type. In this case, GaAs was used for the substrate and GaInNAs was used for the active layer. As growth methods, a solid source MBE method was used for an active layer portion and the MOVPE method, which is advantageous in re-growth, was used for the window region and the p-InP cladding layer. Metal aluminum, metal gallium, and metal indium were used for materials of group-III elements in the solid source molecular-beam MBE method. Moreover, as source materials of group-V elements, nitrogen activated by RF plasma and metal arsenic were used. Moreover, Si was used as n-type dopant and Be was used as p-type dopant. As for materials used in the MOVPE method, they are substantially the same as those already described in Examples 1 and 2. 
         [0040]      FIG. 7  is a cross-sectional view of an laser. First, on an n-GaAs substrates  702 , an n-GaInP cladding layer  704  and a laser part MOW active layer  705  made of GaInNAs/GaAs were successively grown by the MBE method. Next, a mask of a desired shape is formed. After removing an unnecessary MQW region by etching, an Ru-doped GaAs layer  710  was formed by butt-joint regrowth. After removing the mask, a necessary portion was covered again with a mask and a diffraction grating  709  was formed on an Ru-doped GaAs layer. After removing the mask, a p-GaInP cladding layer  708  and a contact layer  706  were grown on entire region, and the crystal growth process was ended. Next, a required portion was protected with a mask and a slant reflector  711  was formed by dry etching. Then, amesa was formed on a stripe by using a mesa mask. Furthermore, a back-surface lens  712  was formed by etching after polishing the substrate. After that, by using an ordinary manufacturing method, a passivation film was formed, planarization was performed by using polyimide, and upper and lower electrodes  707  and  701  were formed to complete the laser. After cleavage, an HR coat film  703  was applied to the back surface of the manufactured laser. 
         [0041]    The 200 μm long laser having a MQW active layer part of 100 μm length was operated with 12 mA current drive at 10 GHz at 100° C. The laser beam was reflected off the slant reflector, and was focused by the back-surface lens, and emitted from the back-surface side. The laser characteristics did not deteriorate even during the long term operation, showing high reliability, and the manufacturing yield of the laser was also high. 
       EXAMPLE 4 
       [0042]    Example 4 is a case where the present invention is applied to an electro-absorption modulator integrated DFB laser. Although the MOVPE method was also used here, growth methods are not limited to it, and other techniques can be used. The materials used were the same as those in Examples 1 and 2.  FIG. 8  is a cross-sectional view of an element. First, a MQW active layer  803  of the laser part made of InGaAlAs was grown on an n-InP substrate  802 . Next, a mask of a desired shape was formed, and an unnecessary MQW active layer  803  was removed by etching. Then, a MQW absorption layer  811  of an electro-absorption modulator part made of InGaAlAs was formed by butt-joint regrowth. After removing the mask, a necessary portion was covered again with a mask, and an InGaAsP-waveguide layer  808  was formed by butt-joint regrowth. After removing the mask, a necessary portion was covered with a mask again, and an InP layer  812  into which Ru was doped was formed by butt-joint regrowth. After removing the mask, a necessary portion was covered with a mask, and a diffraction grating  804  was formed over the MQW active layer  803  of the laser part. After removing the mask, a p-InP cladding layer  805  and contact layers  806  and  809  were grown all over the surface of the laser, and the crystal growth process was ended. After that, by using an ordinary element manufacturing method, a passivation film was formed, planarization was performed by using polyimide, a part of contact layer was removed to separate the modulator portion and laser portion electrically, and upper electrodes  807  and  810  above the laser part and modulator part and a lower electrode  801  were formed to complete the laser. After cleavage, the HR coating (not shown) was applied on one cleaved facet of the manufactured laser. The manufactured laser was operated at a temperature within a range of from −5° C. to 85° C. at 10 GHz without a cooler. The laser characteristics did not deteriorate even during the long term operation, showing high reliability, and the manufacturing yield of the laser was also high. 
         [0043]    Reference numerals used in the drawings of the application is described below. 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 101 
                 lower electrode, 
               
               
                 102 
                 n-InP substrate, 
               
               
                 103 
                 HR coating film, 
               
               
                 104 
                 active layer, 
               
               
                 105 
                 p-InP cladding layer, 
               
               
                 106 
                 contact layer, 
               
               
                 107 
                 upper electrode, 
               
               
                 108 
                 diffraction grating, 
               
               
                 109 
                 InGaAsP layer, 
               
               
                 110 
                 slant reflector, 
               
               
                 111 
                 back-surface lens, 
               
               
                 201 
                 lower electrode, 
               
               
                 202 
                 n-InP substrate, 
               
               
                 203 
                 HR coating film, 
               
               
                 204 
                 active layer, 
               
               
                 205 
                 p-InP cladding layer, 
               
               
                 206 
                 contact layer, 
               
               
                 207 
                 upper electrode, 
               
               
                 208 
                 diffraction grating, 
               
               
                 209 
                 InGaAsP layer, 
               
               
                 210 
                 slant reflectors, 
               
               
                 211 
                 back-surface lens, 
               
               
                 301 
                 n-InP substrate, 
               
               
                 302 
                 active layer, 
               
               
                 303 
                 p-InP cladding layer, 
               
               
                 304 
                 contact layer, 
               
               
                 305 
                 diffraction grating, 
               
               
                 306 
                 InGaAsP layer, 
               
               
                 307 
                 slant reflector, 
               
               
                 308 
                 back-surface lens, 
               
               
                 309 
                 n-type InP substrate, 
               
               
                 310 
                 active layer, 
               
               
                 311 
                 p-InP cladding layer, 
               
               
                 312 
                 contact layer, 
               
               
                 313 
                 diffraction grating, 
               
               
                 314 
                 InGaAsP layer, 
               
               
                 315 
                 slant reflector, 
               
               
                 316 
                 semi-insulating InP buried layer, 
               
               
                 317 
                 back-surface lens, 
               
               
                 501 
                 lower electrode, 
               
               
                 502 
                 n-InP substrate, 
               
               
                 503 
                 HR coating film, 
               
               
                 504 
                 active layer, 
               
               
                 505 
                 p-InP cladding layer, 
               
               
                 506 
                 contact layer, 
               
               
                 507 
                 upper electrode, 
               
               
                 508 
                 diffraction grating, 
               
               
                 509 
                 Ru-doped InGaAsP layer, 
               
               
                 510 
                 slant reflector, 
               
               
                 511 
                 back-surface lens, 
               
               
                 601 
                 lower electrode, 
               
               
                 602 
                 n-InP substrate, 
               
               
                 603 
                 HR coating film, 
               
               
                 604 
                 active layer, 
               
               
                 605 
                 diffraction grating, 
               
               
                 606 
                 contact layer, 
               
               
                 607 
                 upper electrode, 
               
               
                 608 
                 p-InP cladding layer, 
               
               
                 609 
                 Fe-doped InP layer 
               
               
                 610 
                 Ru-doped InP layer, 
               
               
                 611 
                 slant reflector, 
               
               
                 612 
                 n-InP buffer layer, 
               
               
                 613 
                 multilayer reflecting mirror, 
               
               
                 614 
                 back-surface lens, 
               
               
                 701 
                 lower electrode, 
               
               
                 702 
                 n-GaAs substrate, 
               
               
                 703 
                 HR coating film, 
               
               
                 704 
                 an n-GaInP cladding layer, 
               
               
                 705 
                 laser part MOW active layer, 
               
               
                 706 
                 contact layer, 
               
               
                 707 
                 upper electrode, 
               
               
                 708 
                 p-GaInP cladding layer, 
               
               
                 709 
                 diffraction grating, 
               
               
                 710 
                 Ru-doped GaAs layer, 
               
               
                 711 
                 slant reflector, 
               
               
                 712 
                 back-surface lens, 
               
               
                 801 
                 lower electrode, 
               
               
                 802 
                 an n-InP substrate, 
               
               
                 803 
                 active layer of laser, 
               
               
                 804 
                 diffraction grating, 
               
               
                 805 
                 p-InP cladding layer, 
               
               
                 806 
                 contact layer of the laser part, 
               
               
                 807 
                 upper electrodes above the laser part, 
               
               
                 808 
                 waveguide layer, 
               
               
                 809 
                 contact layer of the modulator part, 
               
               
                 810 
                 upper electrodes above the modulator part, 
               
               
                 811 
                 absorption layer of modulator, 
               
               
                 812 
                 Ru-doped InP layer