Abstract:
There is provided a semiconductor laser including (a) an n-type semiconductor substrate, (b) an active layer formed on the n-type semiconductor substrate, (c) a first p-type semiconductor layer formed adjacent to the active layer, (d) an n-type semiconductor layer formed adjacent to the first p-type semiconductor layer, (e) a second p-type semiconductor layer formed adjacent to the n-type semiconductor layer, and (f) a lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer. The semiconductor laser prevents a pnpn thyrister from turning on to thereby ensure sufficient block breakdown voltage even at a high temperature or even when much current is applied thereto.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a semiconductor laser comprised of a pnpn thyrister and including a current strangulation structure, and also to a method of fabricating the same.  
           [0003]    2. Description of the Related Art  
           [0004]    A semiconductor laser including an active layer formed by selective growth and having a pnpn thyrister block structure formed by selective growth can be fabricated without carrying out a step of etching a semiconductor layer. Hence, a width of an active layer can be accurately controlled, ensuring uniformity in characteristic and reproducibility of a semiconductor laser.  
           [0005]    [0005]FIG. 1 illustrates one of conventional semiconductor lasers having such a structure as mentioned above.  
           [0006]    The illustrated semiconductor laser is comprised of an n-InP substrate  701 , an electrode  711  formed on a lower surface of the n-InP substrate  701 , a stripe including an n-InP clad layer  703 , an MQW active layer  704 , and a p-InP clad layer  705 , a p-InP block layer  707  covering the n-InP substrate  701  and the stripe therewith, an n-InP block layer  708  formed on the p-InP block layer  707 , a p-InP clad layer  709  formed on the n-InP block layer  708 , a p-InGaAs cap layer  710  formed on the p-InP clad layer  709 , and an electrode  712  formed on the p-InGaAs cap layer  710 .  
           [0007]    In the illustrated semiconductor laser, since a current is strangulated in the stripe including the MQW active layer  704 , around the stripe is formed a pnpn thyrister block structure comprised of the n-InP substrate  701 , the p-InP block layer  707 , the n-InP block layer  708 , and the p-InP clad layer  709 . The pnpn thyrister block structure prevents a current from running outside the stripe.  
           [0008]    Japanese Unexamined Patent Publication No. 5-67849 has suggested a semiconductor light-emitting device including a p-InP substrate formed with a mesa-stripe, an n-InP block layer, a p-InP buffer layer, an InGaAsP active layer, and n-InP clad layer all deposited on the p-InP substrate, a p-InP buried layer, an n-InP current-blocking layer, and a p-InP current-blocking layer all deposited in a recess formed in the InGaAsP active layer and the n-InP clad layer, an n-InP buried layer covering the n-InP clad layer and the p-InP current-blocking layer therewith, and a pair of electrodes.  
           [0009]    Japanese Unexamined Patent Publication No. 8-330676 has suggested a semiconductor laser including a p-InP substrate, a pair of SiO 2  stripe masks formed on the p-InP substrate in a [ 011 ] direction and spaced away from each other by 1.5 μm, and a multi-layered structure including an active layer, formed in the 1.5 μm-space by MOVPE selective growth.  
           [0010]    Japanese Unexamined Patent Publication No. 9-266349 has suggested a semiconductor laser including a p-InP substrate including a buffer layer, a trapezoidal selective growth portion formed on the p-InP substrate. The selective growth portion includes a p-clad layer, SCH strain MQW layer, and n-clad layer, and acts as a waveguide for a laser having a wavelength of 1.3 μm. Around the selective growth portion are deposited p-InP buried layer, n-InP layer, p-InP layer, and SCH-MQW carrier recombination layer. An n-InP clad buried layer covers the above-mentioned structure, and an n-InGaAsP contact layer is formed on the n-InP clad buried layer.  
           [0011]    However, the above-mentioned conventional semiconductor lasers are accompanied with a problem that turn-on occurs in the pnpn thyrister at a high temperature or when much current is applied to the semiconductor lasers, and hence, it is not always ensured to provide a sufficient block breakdown voltage.  
           [0012]    It is most effective to design a block layer to have a greater thickness in order to enhance a breakdown voltage of a thyrister. However, a thickness of a current-blocking layer has upper limitation in the above-mentioned conventional semiconductor lasers, because of a demand in a waveguide layer to have a small height. As a result, it is quite difficult or almost impossible in the above-mentioned conventional semiconductor lasers to make a thickness of a current-blocking layer greater in order to enhance a breakdown voltage of a thyrister.  
         SUMMARY OF THE INVENTION  
         [0013]    In view of the above-mentioned problem, it is an object of the present invention to provide a semiconductor laser and a method of fabricating the same both of which is capable of preventing occurrence of turn-on in a pnpn thyrister, and providing a sufficient block breakdown voltage even at a high temperature or even when much current is applied to a pnpn thyrister.  
           [0014]    In one aspect of the present invention, there is provided a semiconductor laser including (a) an n-type semiconductor substrate, (b) an active layer formed on the n-type semiconductor substrate, (c) a first p-type semiconductor layer formed adjacent to the active layer, (d) an n-type semiconductor layer formed adjacent to the first p-type semiconductor layer, (e) a second p-type semiconductor layer formed adjacent to the n-type semiconductor layer, and (f) a lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer.  
           [0015]    In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer formed between the n-type substrate and the first p-type semiconductor layer suppresses electron-ejection into the first p-type semiconductor layer. As a result, it is possible to reduce the number of electrons passing through the first p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.  
           [0016]    The lightly doped n-type semiconductor layer contains an impurity at preferably 3×10 17  cm −3  or smaller, and more preferably at 1×10 17  cm −3  or smaller. Such concentration of an impurity more effectively suppresses electron-ejection into the adjacent p-type semiconductor layer, ensuring an increase in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to a thyrister.  
           [0017]    There is no lower limitation in the concentration of an impurity. However, it is preferable that the lightly doped n-type semiconductor layer contains an impurity at such a concentration that there does not occur auto-dope caused by a p-type impurity. For instance, the lightly doped n-type semiconductor layer contains an impurity at preferably 1×10 15  cm −3  or greater.  
           [0018]    The lightly doped n-type semiconductor layer has a thickness of preferably 0.5 μm or greater, and more preferably 1.0 μm or greater.  
           [0019]    If the lightly doped n-type semiconductor layer is too thin, there might occur the tunneling effect in which electrons pass through the lightly doped n-type semiconductor layer. There is no upper limitation in a thickness of the lightly doped n-type semiconductor layer, unless the thickness does not deteriorate a structure of a semiconductor laser.  
           [0020]    For instance, the lightly doped n-type semiconductor layer is designed to contain Si, S or Se as an n-type impurity.  
           [0021]    It is preferable that the lightly doped n-type semiconductor layer extends entirely over p-n junction plane, which would effectively suppress electron-ejection into the adjacent p-type semiconductor layer.  
           [0022]    For instance, the lightly doped n-type semiconductor layer may be formed in the n-type semiconductor substrate, in which case, it is preferable that the lightly doped n-type semiconductor layer has a depth equal to a thickness of the n-type semiconductor substrate.  
           [0023]    For instance, the lightly doped n-type semiconductor layer may be formed on the n-type semiconductor substrate.  
           [0024]    There is further provided a semiconductor laser including (a) a p-type semiconductor substrate, (b) an active layer formed on the p-type semiconductor substrate, (c) a first n-type semiconductor layer formed adjacent to the active layer, (d) a p-type semiconductor layer formed adjacent to the first n-type semiconductor layer, (e) a second n-type semiconductor layer formed adjacent to the p-type semiconductor layer, and (f) a lightly doped n-type semiconductor layer formed between the p-type semiconductor layer and the second n-type semiconductor layer.  
           [0025]    In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer formed between the p-type semiconductor layer and the second n-type semiconductor layer suppresses electron-ejection into the p-type semiconductor layer. As a result, it is possible to reduce the number of electrons passing through the p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.  
           [0026]    The lightly doped n-type semiconductor layer contains an impurity at preferably 3×10 17  cm −3  or smaller, and more preferably 1×10 17  cm −3  or smaller.  
           [0027]    For instance, the lightly doped n-type semiconductor layer may be designed to contain an impurity at 1×10 15  cm −3  or greater.  
           [0028]    There is still further provided a semiconductor laser including (a) a pnpn thyrister, (b) an n-type electrode making electrical contact with an n-type region located at an end of the pnpn thyrister, and (c) a lightly doped n-type semiconductor layer located adjacent to p-n junction plane closer to the n-type electrode among p-n junction planes of the pnpn thyrister.  
           [0029]    In accordance with the above-mentioned semiconductor laser, the lightly doped n-type semiconductor layer located adjacent to p-n junction plane closer to the n-type electrode among p-n junction planes of the pnpn thyrister suppresses electron-ejection into a p-type semiconductor region. As a result, it is possible to reduce the number of electrons passing through the p-type semiconductor layer and charging up in the n-type semiconductor layer, ensuring enhancement in a breakdown voltage of a thyrister even at a high temperature or even when much current is applied to the thyrister.  
           [0030]    In another aspect of the present invention, there is provided μmethod of fabricating a semiconductor laser, including the steps of (a) forming a lightly doped n-type semiconductor layer on or in an n-type semiconductor substrate, (b) ion-implanting an n-type impurity to the lightly doped n-type semiconductor layer to thereby partially turn the lightly doped n-type semiconductor layer into a heavily doped n-type semiconductor layer, (c) forming a light-waveguide layer on the heavily doped n-type semiconductor layer, the light-waveguide layer including an active layer, (d) forming a first p-type semiconductor layer on the lightly doped n-type semiconductor layer, (e) forming an n-type semiconductor layer on the first p-type semiconductor layer, and (f) forming a second p-type semiconductor layer on the light-waveguide layer and the n-type semiconductor layer.  
           [0031]    In accordance with the above-mentioned method, it is possible to fabricate a semiconductor laser having a high breakdown voltage, in the reduced number of fabrication steps at a high fabrication yield.  
           [0032]    The heavily doped n-type semiconductor layer contains an impurity at preferably 5×10 17  to 5×10 18  cm −3 , and more preferably 1×10 18  to 3×10 18  cm −3 .  
           [0033]    The lightly doped n-type semiconductor layer contains an impurity at preferably 3×10 17  cm −3  or smaller, more preferably 1×10 17  cm −3  or smaller, and preferably 1×10 15  cm −3  or greater.  
           [0034]    There is further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a heavily doped n-type semiconductor layer on or in an n-type semiconductor substrate, (b) forming a lightly doped n-type semiconductor layer entirely covering the heavily doped n-type semiconductor layer therewith, (c) at least partially removing the heavily doped n-type semiconductor layer and the lightly doped n-type semiconductor layer to thereby cause the heavily doped n-type semiconductor layer to appear, (d) forming a light-waveguide layer on the heavily doped n-type semiconductor layer, the light-waveguide layer including an active layer, (e) forming a first p-type semiconductor layer on the lightly doped n-type semiconductor layer, (f) forming an n-type semiconductor layer on the first p-type semiconductor layer, and (g) forming a second p-type semiconductor layer on the light-waveguide layer and the n-type semiconductor layer.  
           [0035]    The above-mentioned method makes it possible to fabricate a lightly doped n-type semiconductor layer including fewer defects such as crystal defect.  
           [0036]    For instance, the heavily doped n-type semiconductor layer and the lightly doped n-type semiconductor layer may be partially removed in the step (c) by chemical mechanical polishing or dry etching.  
           [0037]    There is still further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a lightly doped n-type semiconductor layer on an n-type semiconductor substrate, (b) patterning the lightly doped n-type semiconductor layer so that a space is formed in the thus patterned lightly doped n-type semiconductor layer, (c) forming a light-waveguide layer on the n-type semiconductor substrate in the space, the light-waveguide layer including an active layer, (d) forming a first p-type semiconductor layer on the lightly doped n-type semiconductor layer, (e) forming an n-type semiconductor layer on the first p-type semiconductor layer, and (f) forming a second p-type semiconductor layer on the light-waveguide layer and the n-type semiconductor layer.  
           [0038]    There is yet further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a first lightly doped p-type semiconductor layer on or in a p-type semiconductor substrate, (b) forming a light-waveguide layer on the first lightly doped p-type semiconductor layer, the light-waveguide layer including an active layer, (d) forming a first n-type semiconductor layer on the first lightly doped p-type semiconductor layer, (e) forming a first p-type semiconductor layer on the first n-type semiconductor layer, (f) forming an etching stopper layer on the first p-type semiconductor layer, (g) forming a second n-type semiconductor layer on the light-waveguide layer and the etching stopper layer, (h) etching the second n-type semiconductor layer so that a portion of the second n-type semiconductor layer remains only above the light-waveguide layer, and (i) depositing a second lightly doped n-type semiconductor layer on the etching stopper layer around the portion of the second n-type semiconductor layer.  
           [0039]    There is still yet further provided a method of fabricating a semiconductor laser, including the steps of (a) forming a first lightly doped p-type semiconductor layer on or in a p-type semiconductor substrate, (b) forming a light-waveguide layer on the first lightly doped p-type semiconductor layer, the light-waveguide layer including an active layer, (c) forming a first n-type semiconductor layer on the first lightly doped p-type semiconductor layer, (d) forming a p-type semiconductor layer on the first n-type semiconductor layer, (e) forming a second n-type semiconductor layer on the light-waveguide layer and the p-type semiconductor layer, (f) forming a mask on the second n-type semiconductor layer, the mask having an opening located just above the light-waveguide layer, and (g) ion-implanting an n-type impurity into the second n-type semiconductor layer through the opening of the mask.  
           [0040]    The advantages obtained by the aforementioned present invention will be described hereinbelow.  
           [0041]    In accordance with the above-mentioned present invention, since a lightly doped n-type semiconductor layer is formed at a p-n junction plane, it is possible to enhance a block breakdown voltage at a high temperature or when much current is applied to a semiconductor laser, increase a saturated output, and reduce a drive current at a high temperature, ensuring longer lifetime of a semiconductor laser. 
       
    
    
       [0042]    The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]    [0043]FIG. 1 is a cross-sectional view of a conventional semiconductor laser.  
         [0044]    [0044]FIG. 2 is a cross-sectional view of a semiconductor laser in accordance with the first embodiment.  
         [0045]    [0045]FIGS. 3A to  3 F are cross-sectional views of a semiconductor laser, illustrating respective steps of a method of fabricating the same, in accordance with the second embodiment.  
         [0046]    [0046]FIGS. 4A to  4 F are cross-sectional views of a semiconductor laser, illustrating respective steps of a method of fabricating the same, in accordance with the third embodiment.  
         [0047]    [0047]FIGS. 5A to  5 F are cross-sectional views of a semiconductor laser, illustrating respective steps of a method of fabricating the same, in accordance with the fourth embodiment.  
         [0048]    [0048]FIGS. 6A to  6 E are cross-sectional views of a semiconductor laser, illustrating respective steps of a method of fabricating the same, in accordance with the fifth embodiment.  
         [0049]    [0049]FIGS. 7A to  7 C are cross-sectional views of a semiconductor laser, illustrating respective steps of a method of fabricating the same, in accordance with the sixth embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0050]    [0050]FIG. 2 is a cross-sectional view of a semiconductor laser in accordance with the first embodiment.  
         [0051]    The illustrated semiconductor laser is comprised of an n-InP substrate  1 , a lightly doped n-type InP layer  2  (n − -InP), a waveguide layer  6  including an n-InP clad layer  3 , an MQW active layer  4 , and a p-InP clad layer  5 , a p-InP block layer  7  covering the n-InP substrate  701  and the waveguide layer  6  therewith, an n-InP block layer  8  formed on the p-InP block layer  7 , a p-InP buried layer  9  formed on the n-InP block layer  8 , a p-InGaAs cap layer  10  formed on the p-InP buried layer  9 , an n-side electrode  11   a  formed on a lower surface of the n-InP substrate  1 , and a p-side electrode  11   b  formed on the p-InGaAs cap layer  10 .  
         [0052]    The n-InP substrate  1  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 . The lightly doped n-type InP layer  2  has a concentration of an n-type impurity of 1×10 17  cm −3 , a width in the range of 3 to 5 μm, and a thickness in the range of 0.5 to 1 μm. The waveguide layer  6  has a width in the range of 1.2 to 1.6 μm. The p-InP block layer  7  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.6 μm. The n-InP block layer  8  has a concentration of an n-type impurity in the range of 7×10 17  cm −3  to 9×10 17  cm −3 , and a thickness of 0.6 μm. The p-InP buried layer  9  has a concentration of a p-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness of 2.5 μm. The p-InGaAs cap layer  10  has a concentration of a p-type impurity in the range of 1×10 19  cm −3  to 2×10 19  cm −3 , and a thickness of 0.2 μm.  
         [0053]    Since the highly doped n-InP substrate  1  is located just below the MQW active layer  4 , a series resistance to electrons to flow into the MQW active layer  4  is made small. On the other hand, since the p-InP block layer  7  makes contact with the lightly doped n-type InP layer  2 , the number of electrons to flow into the p-InP block layer  7  is made small.  
         [0054]    Since the semiconductor laser in accordance with the first embodiment is formed at the n-InP substrate  1  with a strangulation structure to electrons, a rate of electron-ejection to the p-InP block layer  7  is suppressed, which ensures reduction in the number of electrons which pass through the p-InP current-blocking layer  7  and charging up in the n-InP current-blocking layer  8 . Hence, a breakdown voltage of a thyrister is enhanced even at a high temperature or even when much current is applied to the thyrister.  
       Second Embodiment  
       [0055]    [0055]FIGS. 3A to  3 F illustrate respective steps of a method of fabricating a semiconductor laser, in accordance with the second embodiment.  
         [0056]    First, as illustrated in FIG. 3A, a lightly doped n-type InP layer  2  (n − -InP) is formed on an n-InP substrate  1 . The lightly doped n-type InP layer  2  has a concentration of an n-type impurity of 1×10 17  cm −3  and a thickness in the range of 0.5 to 1 μm. The n-InP substrate  1  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 .  
         [0057]    Then, as illustrated in FIG. 3B, a pair of masks  51  is formed on the n-InP substrate  1  by photolithography. The masks  51  are composed of dielectric material such as SiO 2 . The masks  51  may be composed of SiN in place of SiO 2 . Each of the masks  51  is designed to have a width in the range of 3 to 5 μm. A space between a pair of the masks  51 , that is, a width of an area in which an active layer is to be formed, is in the range of 1.0 to 1.6 μm.  
         [0058]    Then, as illustrated in FIG. 3C, an n-type impurity such as Si is ion-implanted into the lightly doped n-type InP layer  2  through the use of the masks  51 . As a result, the lightly doped n-type InP layer  2  is partially turned into a heavily doped n-type InP region having a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 .  
         [0059]    Then, as illustrated in FIG. 3D, there is formed a waveguide layer including an MQW active layer  4 , an n-InP clad layer  3 , and a p-InP clad layer  5 , on the heavily doped n-type InP region by selective growth.  
         [0060]    The n-InP clad layer  3  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm. The p-InP clad layer  5  has a concentration of a p-type impurity in the range of 5×10 17  cm −3  to 7×10 17  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm.  
         [0061]    The MQW active layer  4  is comprised of (a) an n-InGaAsP SCH layer having a composition of 1.13 μm, a concentration in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness of 60 nm, (b) seven strained InGaAsP quantum well layers having 0.7% compression strain and a thickness of 4.5 nm, (c) an InGaAsP barrier layer having a composition of 1.13 μm and a thickness of 7 nm, and (d) a u-InGaAsP SCH layer having a composition of 1.13 μm and a thickness of 60 nm.  
         [0062]    Then, there are formed masks only on the waveguide layers. Then, as illustrated in FIG. 3E, a p-InP block layer  7  and an n-InP block layer  8  are successively deposited over a resultant around the masks.  
         [0063]    The p-InP block layer  7  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.6 μm. The n-InP block layer  8  has a concentration of an n-type impurity in the range of 7×10 17  cm −3  to 9×10 17  cm −3 , and a thickness of 0.6 μm.  
         [0064]    Then, as illustrated in FIG. 3F, a resultant is covered with a p-InP clad layer  9 , and further with a p + -InGaAs cap layer  10 . The p-InP clad layer  9  has a concentration of a p-type impurity of 1×10 18  cm −3 , and a thickness of 2 μm. The p + -InGaAs cap layer  10  has a concentration of a p-type impurity of 1×10 19  cm −3 , and a thickness of 0.2 μm.  
         [0065]    In accordance with the above-mentioned method, since the masks  51  are used commonly for ion-implantation and selective growth, it is possible to form the heavily doped n-type InP region just below the active layer  4 , and form the lightly doped n-type InP layer  2  just below the p-InP block layer  7  in self-align manner.  
         [0066]    The experiment was conducted to confirm that a semiconductor laser fabricated in accordance with the above-mentioned method had superior output characteristic.  
         [0067]    A semiconductor laser fabricated in accordance with the above-mentioned method was cleaved into a resonator length of 300 μm. An end protection film having a reflectance of 30% and composed of SiO 2  was formed at a front end, and a high reflectance film constituted of SiO 2 /α-Si multiple layers was formed at a rear end of the thus cleaved semiconductor laser.  
         [0068]    Then, the semiconductor laser was mounted on a boron-nitride heat sink in junction-down, and optical output characteristic of the semiconductor laser was measured. The semiconductor laser was oscillated at 1.3 μm, and showed that a threshold value was 7 mA and a slope efficiency was 0.5 W/A both at room temperature, and a threshold value was 15 mA and a slope efficiency was 0.4 W/A both at 85 degrees centigrade, and that a drive current at optical output of 14 mW was 60 mA.  
         [0069]    In a conventional semiconductor laser where a region located just below a p-InP block layer had a p-type impurity concentration of 1×10 18  cm −3 , a drive current at optical output of 14 mW was 70 mA.  
         [0070]    In comparison with the conventional semiconductor laser, it was confirmed that the semiconductor laser in accordance with the instant embodiment had superior high-temperature characteristics.  
       Third Embodiment  
       [0071]    [0071]FIGS. 4A to  4 F illustrate respective steps of a method of fabricating a semiconductor laser, in accordance with the third embodiment.  
         [0072]    Then, as illustrated in FIG. 4A, a pair of masks  51  is formed on an n-InP substrate  1  by photolithography. The n-InP substrate  1  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 . The masks  51  are composed of dielectric material such as SiO 2 . The masks  51  may be composed of SiN in place of SiO 2 . Each of the masks  51  is designed to have a width in the range of 5 to 8 μm. A space between a pair of the masks  51 , that is, a width of an area in which an active layer is to be formed, is in the range of 1.2 to 1.6 μm.  
         [0073]    Then, as illustrated in FIG. 4B, heavily doped n-type InP regions  21  (n + -InP) are formed on n-InP substrate  1  by selective growth. The heavily doped n-type InP regions  21  have a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 .  
         [0074]    After etching the masks  51  off, as illustrated in FIG. 4C, a resultant is covered entirely with a lightly doped n-InP (n − -InP) layer  2 . The lightly doped n-InP layer  2  have a concentration of an n-type impurity of 1×10 17  cm −3 .  
         [0075]    Then, as illustrated in FIG. 4D, a resultant is planarized by mechanical chemical polishing (CMP) until the heavily doped n-type InP regions  21  appear. Then, a λ/4-shift diffraction lattice  22  is formed on one of the exposed heavily doped n-type InP regions  21  by interference exposure or EB exposure. The λ/4-shift diffraction lattice  22  has a pitch of about 203 nm and a depth of 50 nm.  
         [0076]    Then, masks  51  composed of SiO 2  are formed again by photolithography. Each of the masks  51  is designed to have a width in the range of 3 to 5 μm. A space between a pair of the masks  51 , that is, a width of an area in which an active layer is to be formed, is in the range of 1.2 to 1.6 μm. The masks  51  are formed so that the space between the adjacent masks overlaps the heavily doped n-type InP layer  21 .  
         [0077]    Then, as illustrated in FIG. 4E, there is formed a waveguide layer on the heavily doped n-type InP layer  21  by selective growth. The waveguide layer is constituted of an n-InGaAsP guide layer  23 , an n-InP spacer layer  24 , a MQW active layer  4 , and a p-InP clad layer  5 .  
         [0078]    The n-InGaAsP guide layer  23  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , a composition of 1.13 μm, and a thickness of 60 nm. The n-InP spacer layer  24  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness of 60 nm. The p-InP clad layer  5  has a concentration of a p-type impurity in the range of 5×10 17  cm −3  to 7×10 17  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm.  
         [0079]    A stand-by time until a temperature reaches a crystal growth temperature, and flow rates of PH 3  and AsH 3  are controlled so that the diffraction lattice  22  has a height of 27 nm after having been buried.  
         [0080]    The MQW active layer  4  is comprised of (a) an n-InGaAsP SCH layer having a composition of 1.13 μm, a concentration in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness of 20 nm, (b) ten strained InGaAsP quantum well layers having 0.9% compression strain and a thickness of 4 nm, (c) an InGaAsP barrier layer having a composition of 1.13 μm and a thickness of 7 nm, and (d) a u-InGaAsP SCH layer having a composition of 1.13 μm and a thickness of 60 nm.  
         [0081]    Then, there are formed masks only on the waveguide layers. Then, as illustrated in FIG. 4F, a p-InP block layer  7  and an n-InP block layer  8  are successively deposited over a resultant around the masks.  
         [0082]    The p-InP block layer  7  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.4 μm. The n-InP block layer  8  has a concentration of an n-type impurity in the range of 7×10 17  cm −3  to 9×10 17  cm −3 , and a thickness of 0.5 μm.  
         [0083]    Then, as illustrated in FIG. 4F, a resultant is covered with a p-InP clad layer  9 , and further with a p + -InGaAs cap layer  10 . The p-InP clad layer  9  has a concentration of a p-type impurity of 1×10 18  cm −3 , and a thickness of 2 μm. The p + -InGaAs cap layer  10  has a concentration of a p-type impurity of 1×10 19  cm −3 , and a thickness of 0.2 μm.  
         [0084]    The experiment was conducted to confirm that a semiconductor laser fabricated in accordance with the above-mentioned method had superior output characteristic.  
         [0085]    A semiconductor laser fabricated in accordance with the above-mentioned method was cleaved into a resonator length of 300 μm so that λ/4 shift position is located at the center of a pellet. Opposite ends of the thus cleaved semiconductor laser are coated with AR coat composed of a SiN film. Then, the semiconductor laser was mounted on an aluminum-nitride heat sink in junction-down, and optical output characteristic of the semiconductor laser was measured. The semiconductor laser showed that a threshold value was 10 mA and a slope efficiency was 0.35 W/A both at room temperature, and a threshold value was 20 mA and a slope efficiency was 0.25 W/A both at 85 degrees centigrade, and that a drive current at optical output of 10 mW was 65 mA.  
         [0086]    There occurred single mode oscillation at an oscillation wavelength of 1.3 μm and at a sub-mode suppression ratio of 50 dB.  
         [0087]    In a conventional semiconductor laser where a region located just below a p-InP block layer had a p-type impurity concentration of 1×10 18  cm −3 , a drive current at optical output of 10 mW was 80 mA.  
         [0088]    In comparison with the conventional semiconductor laser, it was confirmed that the semiconductor laser in accordance with the instant embodiment had superior high-temperature characteristics.  
       Fourth Embodiment  
       [0089]    [0089]FIGS. 5A to  5 F illustrate respective steps of a method of fabricating a semiconductor laser, in accordance with the fourth embodiment.  
         [0090]    First, as illustrated in FIG. 5A, a lightly doped n-type InP layer  2  (n − -InP) is formed on a heavily doped n-InP substrate  1 . The lightly doped n-type InP layer  2  has a concentration of an n-type impurity of 1×10 17  cm −3  and a thickness in the range of 0.5 to 1 μm. The heavily doped n-InP substrate  1  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 .  
         [0091]    Then, as illustrated in FIG. 5B, a pair of masks  51  is formed on the n-InP substrate  1  by photolithography. The masks  51  are composed of dielectric material such as SiO 2 . The masks  51  may be composed of SiN in place of SiO 2 . Each of the masks  51  is designed to have a width in the range of 3 to 5 μm. A space between a pair of the masks  51 , that is, a width of an area in which an active layer is to be formed, is in the range of 1.2 to 1.6 μm.  
         [0092]    Then, as illustrated in FIG. 5C, the lightly doped n-type InP layer  2  is partially removed by dry etching through the use of the masks  51 .  
         [0093]    Then, as illustrated in FIG. 5D, there is formed a waveguide layer including an n-InP clad layer  3 , an MQW active layer  4 , and a p-InP clad layer  5 , on the exposed area of the n-InP substrate  1  by selective growth.  
         [0094]    The n-InP clad layer  3  has a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm. The p-InP clad layer  5  has a concentration of a p-type impurity in the range of 5×10 17  cm −3  to 7×10 17  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm.  
         [0095]    The MQW active layer  4  is comprised of (a) an n-InGaAsP SCH layer having a composition of 1.13 μm, a concentration in the range of 1×10 18  cm −3  to 2×10 18  cm −3 , and a thickness of 33 nm, (b) five strained InGaAsP quantum well layers having 0.8% compression strain and a thickness of 4.5 nm, (c) an InGaAsP barrier layer having a composition of 1.2 μm and a thickness of 7 nm, and (d) a u-InGaAsP SCH layer having a composition of 1.13 μm and a thickness of 33 nm.  
         [0096]    Then, there are formed masks only on the waveguide layers. Then, as illustrated in FIG. 5E, a p-InP block layer  7  and an n-InP block layer  8  are successively deposited over a resultant around the masks.  
         [0097]    The p-InP block layer  7  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.6 μm. The n-InP block layer  8  has a concentration of an n-type impurity in the range of 7×10 17  cm −3  to 9×10 17  cm −3 , and a thickness of 0.6 μm.  
         [0098]    Then, as illustrated in FIG. 5F, a resultant is covered with a p-InP clad layer  9 , and further with a p + -InGaAs cap layer  10 . The p-InP clad layer  9  has a concentration of a p-type impurity of 1×10 18  cm −3 , and a thickness of 2 μm. The p + -InGaAs cap layer  10  has a concentration of a p-type impurity of 1×10 19  cm −3 , and a thickness of 0.2 μm.  
         [0099]    In accordance with the above-mentioned method, since the masks  51  are used commonly for dry etching and selective growth, it is possible to form the heavily doped n-type InP layer just below the active layer  4 , and form the lightly doped n-type InP layer  2  just below the p-InP block layer  7  in self-align manner.  
         [0100]    The experiment was conducted to confirm that a semiconductor laser fabricated in accordance with the above-mentioned method had superior output characteristic.  
         [0101]    A semiconductor laser fabricated in accordance with the above-mentioned method was cleaved into a resonator length of 1200 μm. An AR film having a reflectance of 6% and composed of SiO 2  was formed at a front end, and a high reflectance film having a reflectance of 90% and constituted of SiO 2 /α-Si multiple layers was formed at a rear end of the thus cleaved semiconductor laser.  
         [0102]    Then, the semiconductor laser was mounted on a boron-nitride heat sink in junction-down, and optical output characteristic of the semiconductor laser was measured. The semiconductor laser was oscillated at 1.48 μm, and there was obtained an optical output of 210 W at a drive current of 500 mA at room temperature.  
         [0103]    In a conventional semiconductor laser where a region located just below a p-InP block layer had a p-type impurity concentration of 1×10 18  cm −3 , an optical output was 180 mW.  
         [0104]    In comparison with the conventional semiconductor laser, it was confirmed that the semiconductor laser in accordance with the instant embodiment had superior high-output characteristics. The reason for this is that a breakdown voltage of the thyrister is enhanced because electrons are disallowed to enter the p-InP block layer  7  even when much current is applied to the thyrister.  
       Fifth Embodiment  
       [0105]    [0105]FIGS. 6A to  6 E illustrate respective steps of a method of fabricating a semiconductor laser, in accordance with the fifth embodiment.  
         [0106]    First, as illustrated in FIG. 6A, a lightly doped p-type InP layer  102  (p − -InP) is formed on a heavily doped p-InP substrate  101 . The lightly doped p-type InP layer  102  has a concentration of a p-type impurity of 1×10 17  cm −3  and a thickness of 2 μm. The heavily doped p-InP substrate  101  has a concentration of a p-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 .  
         [0107]    Then, a pair of masks (not illustrated) is formed on the lightly doped p-InP layer  102  by photolithography. The masks are composed of dielectric material such as SiO 2  and SiN. Each of the masks is designed to have a width in the range of 3 to 5 μm. A space between a pair of the masks, that is, a width of an area in which an active layer is to be formed, is in the range of 1.2 to 1.6 μm.  
         [0108]    Then, there is formed a waveguide layer including a p-InP clad layer  103 , an MQW active layer  104 , and an n-InP clad layer  105 , on the lightly doped p-InP layer  102  by selective growth.  
         [0109]    The p-InP clad layer  103  has a concentration of a p-type impurity of 7×10 18  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm. The n-InP clad layer  105  has a concentration of a p-type impurity of 1×10 18  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm.  
         [0110]    The MQW active layer  104  is comprised of (a) a u-InGaAsP SCH layer having a composition of 1.13 μm, a concentration of 7×10 17  cm −3 , and a thickness of 33 nm, (b) five strained InGaAsP quantum well layers having 1% compression strain and a thickness of 4.5 nm, (c) an InGaAsP barrier layer having a composition of 1.2 μm and a thickness of 7 nm, and (d) an n-InGaAsP SCH layer having a composition of 1.13 μm and a thickness of 33 nm.  
         [0111]    Then, there are formed masks only on the waveguide layers. Then, a p-InP block layer  107 , an n-InP block layer  108 , a p-InP block layer  109 , and a p-InGaAsP etching stopper layer  110  are successively deposited over a resultant around the masks.  
         [0112]    The p-InP block layer  107  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.2 μm. The n-InP block layer  8  has a concentration of an n-type impurity in the range of 7×10 17  cm −3  to 9×10 17  cm −3 , and a thickness of 0.6 μm. The p-InP block layer  109  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.6 μm. The p-InGaAsP etching stopper layer has a composition of 1.10 μm and a thickness of 0.1 μm.  
         [0113]    Then, a resultant is covered with an n-InP clad layer  111 . The n-InP clad layer  111  has a concentration of an n-type impurity of 1×10 18  cm −3 , and a thickness of 2 μm. Thus, there is completed such a product as illustrated in FIG. 6A.  
         [0114]    Then, as illustrated in FIG. 6B, a mask  151  composed of SiO 2  and having a width of 2 μm is formed on the n-InP clad layer  111  above the MQW active layer  104 .  
         [0115]    Then, the n-InP clad layer  111  is partially etched through the use of the mask  151 , as illustrated in FIG. 6C. Since the p-InGaAsP etching stopper layer  110  is formed on the p-InP block layer  109 , the p-InP block layer  109  is not etched.  
         [0116]    Then, as illustrated in FIG. 6D, a lightly doped n-InP layer  112  is deposited all over, a resultant through the use of the mask  151  so that a resultant is almost planarized.  
         [0117]    Then, as illustrated in FIG. 6E, the mask  151  is removed.  
         [0118]    The experiment was conducted to confirm that a semiconductor laser fabricated in accordance with the above-mentioned method had superior output characteristic.  
         [0119]    A semiconductor laser fabricated in accordance with the above-mentioned method was cleaved into a resonator length of 900 μm. An AR film having a reflectance of 6% and composed of SiO 2  was formed at a front end, and a high reflectance film having a reflectance of 90% and constituted of SiO 2 /α-Si multiple layers was formed at a rear end of the thus cleaved semiconductor laser.  
         [0120]    Then, the semiconductor laser was mounted on a boron-nitride heat sink in junction-down, and optical output characteristic of the semiconductor laser was measured. The semiconductor laser was oscillated at 1.55 μm, and there was obtained an optical output of 300 W at a pulse drive current of 1A at room temperature, wherein a pulse width was 1 nsec, and a duty ratio was 1%. The reason for this is that a breakdown voltage of the thyrister is enhanced because electrons are disallowed to enter the p-InP block layer  109  even when much current is applied to the thyrister.  
       Sixth Embodiment  
       [0121]    [0121]FIGS. 7A to  7 C illustrates respective steps of a method of fabricating a semiconductor laser, in accordance with the sixth embodiment.  
         [0122]    First, as illustrated in FIG. 7A, a lightly doped p-type InP layer  102  (p − -InP) is formed on a heavily doped p-InP substrate  101 . The lightly doped p-type InP layer  102  has a concentration of a p-type impurity of 7×10 17  cm −3  and a thickness of 2 μm. The heavily doped p-InP substrate  101  has a concentration of a p-type impurity in the range of 1×10 18  cm −3  to 3×10 18  cm −3 .  
         [0123]    Then, a pair of masks (not illustrated) is formed on the lightly doped p-InP layer  102  by photolithography. The masks are composed of dielectric material such as SiO 2  and SiN. Each of the masks is designed to have a width in the range of 3 to 5 μm. A space between a pair of the masks, that is, a width of an area in which an active layer is to be formed, is in the range of 1.2 to 1.6 μm.  
         [0124]    Then, there is formed a waveguide layer including a p-InP clad layer  103 , an MQW active layer  104 , and an n-InP clad layer  105 , on the lightly doped p-InP layer  102  by selective growth.  
         [0125]    The p-InP clad layer  103  has a concentration of a p-type impurity of 7×10 18  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm. The n-InP clad layer  105  has a concentration of a p-type impurity of 1×10 18  cm −3 , and a thickness in the range of 0.1 μm to 0.2 μm.  
         [0126]    The MQW active layer  104  is comprised of (a) a u-InGaAsP SCH layer having a composition of 1.05 μm, a concentration of 7×10 17  cm −3 , and a thickness of 33 nm, (b) five strained InGaAsP quantum well layers having 1% compression strain and a thickness of 4.5 nm, (c) an InGaAsP barrier layer having a composition of 1.13 μm and a thickness of 7 nm, and (d) an n-InGaAsP SCH layer having a composition of 1.05 μm and a thickness of 33 nm.  
         [0127]    Then, there are formed masks only on the waveguide layers. Then, a p-InP block layer  107 , an n-InP block layer  108 , and a p-InP block layer  109  are successively deposited over a resultant around the masks.  
         [0128]    The p-InP block layer  107  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.2 μm. The n-InP block layer  8  has a concentration of an n-type impurity in the range of 7×10 17  cm −3  to 9×10 17  cm −3 , and a thickness of 0.6 μm. The p-InP block layer  109  has a concentration of a p-type impurity in the range of 4×10 17  cm −3  to 6×10 17  cm −3 , and a thickness of 0.6 μm.  
         [0129]    Then, a resultant is covered with an n-InP clad layer  111 . The n-InP clad layer  111  has a concentration of an n-type impurity of 1×10 17  cm −3 , and a thickness of 2 μm. Thus, there is completed such a product as illustrated in FIG. 7A.  
         [0130]    Then, as illustrated in FIG. 7B, a mask  152  composed of SiO 2  and having a width of 2 μm is formed on the n-InP clad layer  111  by photolithography. The mask  152  is designed to have an opening  152   a  located just above the MQW active layer  104 .  
         [0131]    Then, as illustrated in FIG. 7C, an n-type impurity such as Si is ion-implanted into the n-InP clad layer  111  through the opening  152   a  of the mask  152  to thereby turn a portion of the n-InP clad layer  111  located just above the MQW active layer  104 , into a heavily doped portion having a concentration of an n-type impurity in the range of 1×10 18  cm −3  to 2×10 18  cm −3 .  
         [0132]    Thereafter, the mask  152  is removed. Thus, there is completed the semiconductor laser in accordance with the sixth embodiment.  
         [0133]    The experiment was conducted to confirm that a semiconductor laser fabricated in accordance with the above-mentioned method had superior output characteristic.  
         [0134]    A semiconductor laser fabricated in accordance with the above-mentioned method was cleaved into a resonator length of 900 μm. An AR film having a reflectance of 6% and composed of SiO 2  was formed at a front end, and a high reflectance film having a reflectance of 90% and constituted of SiO 2 /α-Si multiple layers was formed at a rear end of the thus cleaved semiconductor laser.  
         [0135]    Then, the semiconductor laser was mounted on a boron-nitride heat sink in junction-down, and optical output characteristic of the semiconductor laser was measured. The semiconductor laser was oscillated at 1.3 μm, and there was obtained an optical output of 400 W at a pulse drive current of 1A at room temperature, wherein a pulse width was 1 nsec, and a duty ratio was 1%. The reason for this is that a breakdown voltage of the thyrister is enhanced because electrons are disallowed to enter the p-InP block layer  109  even when much current is applied to the thyrister.  
         [0136]    While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.  
         [0137]    The entire disclosure of Japanese Patent Application No. 10-168601 filed on Jun. 16, 1998 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.