Abstract:
A semiconductor device having a GaInNAs quantum well structure including a plurality of barrier layers, in which the emission wavelength of the device can be controlled by varying the thicknesses and compositions of the barrier layers, a semiconductor laser using the semiconductor device, and methods of manufacturing the same are provided. The semiconductor laser includes a GaAs-based substrate, a quantum well structure formed on the GaAs-based substrate, a cladding layer surrounding the quantum well structure, and a pair of electrodes electrically connected to the cladding layer. The quantum well structure include a quantum well layer, a pair of first barrier layers facing each other with the active region therebetween, and a pair of second barrier layers adjacent to the respective first barrier layers. Optical quality degradation in a long wavelength range, which arises with common quantum well structures, and emission wavelength shifting to a shorter wavelength range, which occurs when a GaInNAs quantum well structure is thermally treated, can be prevented.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No. 2004-1805, filed on Jan. 10, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor device, and more particularly, to a semiconductor device having a quantum well structure the emission wavelength of which can be adjusted by varying the thicknesses and compositions of a plurality of barrier layers, a semiconductor laser using the semiconductor device, and methods of manufacturing the semiconductor device and the semiconductor laser.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, optics have been actively studied in order to develop high-speed data communications technology with various applications including laser printers, optical image storage, underground optical cable systems, and optical communications. For example, owing to such development in the field of optical communications, large antennas established on the ground to transmit electromagnetic waves through the air have been replaced with underground optical cables that transmit a large amount of information in the form of optical signals.  
         [0006]     In response to increased demand for a high-speed, inexpensive communication system, an optical fiber with an optical transmission band of longer wavelengths has been developed. Currently, an optical fiber that can be used in a wavelength range from 1.3 μm to 1.5 μm is under development. To implement high-speed information transmission using optical fibers, information needs to be properly converted into an optical signal. To this end, a laser oscillation signal having a wavelength within an optical transmission band of the optical fiber is required. Accordingly, efforts have been made to improve a laser diode in order to oscillate a laser signal having a wavelength within an optical transmission band of the optical fiber.  
         [0007]     Such efforts have involved varying the internal composition and interfacial structures of a device for performance enhancement, size reduction and to reduce heat generation and power consumption. One result is the development of a GaAs-based vertical cavity surface emitting laser (VCSEL) diode that costs low and can be easily combined with an optical fiber.  
         [0008]     A study finding that long-wavelength laser oscillation can be achieved by adding nitrogen to a GaAs-based VCSEL is reported by M. Kondow, et al. in February, 1996 (Jpn. J. Appl. Phys., Vol. 35 (1996), pp. 1273-1275, Part 1, No. 2B, “GaInNAs: A Novel Material for Long-Wavelength-Range Laser Diodes with Excellent High-Temperature Performance”).  
         [0009]     After the publication of Kondow&#39;s paper disclosing that long-wavelength emission can be achieved by adding a small amount of nitrogen to the InGaAs material, due to the requirement for optical communications parts to be used in a metro area network (MAN), 1.3 μm-wavelength semiconductor lasers using GaInNAs have been actively developed. However, if GaAs is used for a barrier and GaInNAs is used for a well, the emission wavelength is shifted to a longer wavelength band as the concentration of nitrogen in the quantum well structure increases, but optical characteristics dramatically deteriorate. Accordingly, a quantum well with excellent emission efficiency even in the wavelength band of 1.25 μm can be obtained with such a GaAs/GaInNAs quantum well structure.  
         [0010]     As a possible solution to the problem described above, a semiconductor laser with a GaAs/GaInNAs quantum well structure having a quantum well barrier composed of GaNAs instead of GaAs was suggested (IEEE, LEOS2001 Annual meeting [proceeding Vol. pp. 12-13], “Long wavelength GaInNaAs ridge waveguide lasers with GaNAs barriers” by J. Harris, et al. The semiconductor laser suggested by Harris, et al. uses GaNAs as a barrier to reduce an energy gap between the barrier and the quantum well and shift the emission wavelength of the quantum well to a longer wavelength band. In this structure, as a layer of GaNAs becomes thicker, the emission wavelength of the quantum well made of GaInNAs is shifted to a longer wavelength range.  
         [0011]     However, when a quantum well barrier is made of GaNAs, as in the GaAs/GaInNAs quantum well structure, the crystal quality rapidly deteriorates as the concentration of nitrogen increases or the quantum well barrier gets thicker.  
         [0012]     In order to solve this problem, a. 1.3-μm semiconductor laser that uses GaP with tensile strength as a barrier is suggested in Applied Physics Letters Vol. 83, No. 1, “High-Performance and High-Temperature Continuous-Wave-Operation 1300 nm InGaAsN Quantum Well Lasers by Organometallic Vapor Phase Epitaxy” by N. Tansu, et al.  
         [0013]     According to N. Tansu, et al., when a Group-III semiconductor material is grown on a GaAs semiconductor substrate, the band gap can be adjusted according to the ratio of As/P, and tensile strain is easy to control. However, in a deposition process using metal-organic chemical vapor deposition (MOCVD), since an As source and a P source are supplied to the same reaction vessel, the inside of the reaction vessel is easily contaminated. Therefore, reproducible deposition is not ensured.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a semiconductor device having a quantum well structure with an emission wavelength of at least 1.3 μm and a method of manufacturing the semiconductor device.  
         [0015]     The present invention also provides a vertical cavity surface emitting laser (VCSEL) having a quantum well structure with an emission wavelength of at least 1.3 μm and a method of manufacturing the VCSEL.  
         [0016]     The present invention further provides an edge-emitting semiconductor laser having a quantum well structure with an emission wavelength of at least 1.3 μm and a method of manufacturing the edge-emitting semiconductor laser.  
         [0017]     According to an aspect of the present invention, there is provided a semiconductor device comprising: a GaAs-based substrate; and a quantum well structure formed on the GaAs-based substrate and including a quantum well layer, a pair of first barrier layers facing each other with the quantum well layer therebetween, and a pair of second barrier layers adjacent to the respective first barrier layers.  
         [0018]     According to another aspect of the present invention, there is provided an edge-emitting semiconductor laser comprising: a GaAs-based substrate; a quantum well structure formed on the GaAs-based substrate; a cladding layer surrounding the quantum well structure; and a pair of electrodes electrically connected to the cladding layer, wherein the quantum well structure comprises a quantum well layer, a pair of first barrier layers facing each other with the quantum well layer therebetween, and a pair of second barrier layers adjacent to the respective first barrier layers.  
         [0019]     According to another aspect of the present invention, there is provided VCSEL comprising: a GaAs-based substrate; a first distributed Bragg reflection region formed on the GaAs-based substrate; a quantum well structure formed on the first DBR (distributed Bragg reflection) region; a second DBR region formed on the quantum well structure; and a pair of electrodes electrically connected to the first and second DBR regions, wherein the quantum well structure comprises a quantum well layer, a pair of first barrier layers facing each other with the quantum well layer therebetween, and a pair of second barrier layers adjacent to the respective first barrier layers.  
         [0020]     According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, the method comprising: preparing a GaAs-based substrate; forming a second lower barrier layer on the GaAs-based substrate; forming a first lower barrier layer on the second lower barrier layer; forming a quantum well layer on the first lower barrier layer; forming a first upper barrier layer on the quantum well structure; and forming a second upper barrier layer on the first upper barrier layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0022]      FIGS. 1 and 2  are a cross-sectional view and an energy band diagram, respectively, illustrating an edge-emitting semiconductor laser according to an embodiment of the present invention;  
         [0023]      FIGS. 3 and 4  are a cross-sectional view and an energy band diagram, respectively, illustrating a quantum well structure according to another embodiment of the present invention;  
         [0024]      FIGS. 5 and 6  are a cross-sectional view and an energy band diagram, respectively, illustrating a quantum well structure according to another embodiment of the present invention;  
         [0025]      FIGS. 7 and 8  are a cross-sectional view and an energy band diagram, respectively, illustrating a quantum well structure according to another embodiment of the present invention;  
         [0026]      FIGS. 9 and 10  are cross-sectional views illustrating a vertical cavity surface emitting laser and it&#39;s active region according to another embodiment of the present invention;  
         [0027]      FIG. 11  is a graph illustrating the wavelength of light emitted from a quantum well structure according to the present invention when the thickness of a InGaAs layer is fixed and the thickness of a GaNAs layer is varied;  
         [0028]      FIG. 12  is a graph illustrating the wavelength of light emitted from another quantum well structure according to the preset invention when the thickness of the GaNAs layer is fixed and the thickness of the InGaAs layer is varied;  
         [0029]      FIG. 13  is a graph of an emission wavelength versus the amount of indium (In) in the InGaAs layer; and  
         [0030]      FIG. 14  is a graph of an emission wavelength versus the amount of nitrogen (N) in the GaNAs layer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     Embodiments of a semiconductor structure having a quantum well structure with dual barrier layers, a semiconductor laser employing the semiconductor structure, and a method of manufacturing the semiconductor laser will be described in detail with reference to the accompanying drawings. In the drawings, common elements are denoted by identical reference numerals.  
         [0032]      FIG. 1  is a cross-sectional view illustrating an edge-emitting semiconductor laser according to an embodiment of the present invention. As shown in  FIG. 1 , an edge-emitting semiconductor laser  100  includes a semiconductor substrate  104 , an n-type electrode  102  formed on a lower surface of the semiconductor substrate  104 , a lower cladding layer  106 A formed on an upper surface of the semiconductor substrate  104 , an active region  110  formed on the lower cladding layer  106 A, an upper cladding layer  106 B formed on the active region  110 , a contact layer  120  formed on the upper cladding layer  106 B, and a p-type electrode  126  formed on the contact layer  120 .  
         [0033]     In addition, the active region  110  includes a central barrier layer  112 , a lower quantum well layer  114 A, an upper quantum well layer  114 B, a first lower barrier layer  116 A, a first upper barrier layer  116 B, a second lower barrier layer  118 A, and a second upper barrier layer  118 B. The central barrier layer  112  is formed of GaAs in the middle of the active region  110 . The second lower barrier layer  118 A, the first lower barrier layer  116 A, and the lower quantum well layer  114 A are formed sequentially between the central barrier layer  112  and the lower cladding layer  106 A. The upper quantum well layer  114 B, the first upper barrier layer  116 B, and the second upper barrier layer  118 B are formed sequentially between the central barrier layer  112  and the upper cladding layer  106 B.  
         [0034]     The semiconductor substrate  104  is made of an n-type GaAs-based semiconductor material. Various layers may be grown on the semiconductor substrate  104  to easily form a GaAs-based quantum well. The lower cladding layer  106 A is n-type and is formed to a thickness of 18,000 Å using, for example, AlGaAs. The upper cladding layer  106 B is p-type and is formed to a thickness of 18,000 Å using, for example, AlGaAs.  
         [0035]     The contact layer  120  is p-type and is formed to a thickness of 800 Å using, for example, GaAs. The n-type electrode  102  and the p-type electrode  126  are used to excite an active region  110 . The n-type electrode  102  is made of AuGe and the p-type electrode  126  is made of Ti.  
         [0036]     The edge-emitting semiconductor laser  100  according to a first embodiment of the present invention is a striped type. In order to apply a current across striped regions of the active region  110 , an insulating layer  124  made of SiO 2  is formed on the contact layer  120 , and then the insulating layer  124  is patterned as stripes.  
         [0037]     Although not illustrated in the drawings, in order to improve an ohmic contact between the p-type electrode  126  and the p-type contact layer  120 , a metal contact layer formed of Ti or Pt or as a stack of Ti and Pt, may be further formed. In order to improve an ohmic contact between the n-type electrode  102  and the semiconductor substrate  104 , a metal contact layer formed of Ni or Au or as a stack of Ni and Au, may be further included.  
         [0038]     In the first embodiment of the present invention, the p-type electrode  126  of the edge-emitting semiconductor laser is designed to apply a current across striped regions of the active region. However, the p-type electrode  126  of the edge-emitting semiconductor laser can be designed to apply a current across the entire active region. In addition, although the active region  110  is not formed in the shape of stripes, the edge-emitting semiconductor laser is configured to have the active region  110  match the shape of the p-type electrode  126  formed on an open portion of the insulating layer  124 .  
         [0039]      FIG. 2  is an energy band diagram of the edge-emitting semiconductor laser according to the first embodiment of the present invention.  
         [0040]     As shown in  FIG. 2 , the lower quantum well layer  114 A and the upper quantum well layer  114 B, which are used in the active region  110  of the edge-emitting semiconductor laser  100  according to the first embodiment of the present invention, are made of Ga x In 1-x N y As 1-y  where x and y are greater than 0 and less than 1 to a thickness of 2-10 nm. In the first embodiment of the present invention, x is 0.65 and y is 0.01.  
         [0041]     Meanwhile, the first lower barrier layer  116 A and the first upper barrier layer  116 B are made of In x Ga 1-x As where x is greater than 0 and less than 1 to a thickness of 0.1-50 nm. In the first embodiment of the present invention, x is 0.35.  
         [0042]     In addition, the second lower barrier layer  118 A and the second upper barrier layer  118 B are made of GaN x As 1-x  where x is greater than 0 and less than 1 to a thickness of 0.1-20 nm. In the first embodiment of the present invention, x is 0.02.  
         [0043]     Additionally, the central barrier layer  112  is made of GaAs to a thickness of 0-50 nm.  
         [0044]     According to the first embodiment of the present invention, the wavelength of a laser beam emitted in the lower quantum well layer  114 A and the upper quantum well layer  114 B of the active region  110  may be controlled to be at least 1.2 μm by varying the composition and the thickness of the first barrier layers  116 A and  116 B and the second barrier layers  118 A and  118 B. In addition, the degree and form of a compressive strain induced in the lower quantum well layer  114 A and the upper quantum well layer  114 B may be controlled by varying the composition of indium (In) in the first lower and upper barrier layers  116 A and  116 B.  
         [0045]     In addition, the degree and form of a tensile strain induced in the lower quantum well layer  114 A and the upper quantum well layer  114 B may be controlled by varying the composition of N in the second lower and upper barrier layers  118 A and  118 B. Also, the degree and form of the compressive strain or tensile strain induced in the lower quantum well layer  114 A and the upper quantum well layer  114 B can be controlled by varying the thickness of the first barrier layers  116 A or the second barrier layers  116 B.  
         [0046]     According to the first embodiment of the present invention, the wavelength of the laser beam may be controlled by varying the composition or thickness of the first barrier layers  116 A and  116 B and the second barrier layers  118 A and  118 B. Accordingly, even if the crystalline form of the quantum well layers  114 A and  114 B is deteriorated by the first barrier layers  116 A and  118 B, the crystalline form of the quantum well layers  114 A and  114 B can be dramatically improved by appropriately deforming the second barrier layers  118 A and  118 B.  
         [0047]     As a result, in the first embodiment of the present invention, the wavelength of the laser beam emitted in the quantum well layers may be controlled to be 100 nm or greater without deterioration of optical characteristics by varying the composition and thickness of the first barrier layers and the second barrier layers that have the same structures as the quantum well layers.  
         [0048]     Although in the first embodiment of the present invention the quantum well layers are formed as a dual layer, an edge-emitting semiconductor layer including a plurality of quantum well layers, i.e., more than two quantum well layers, between the lower and upper cladding layers  106 A and  106 B may be manufactured.  
         [0049]      FIGS. 3 and 4  are a cross-sectional view and an energy band diagram, respectively, of a quantum well structure according to another embodiment of the present invention. In the second embodiment, the configurations and functions of all components except for the active region are identical to the first embodiment.  
         [0050]     The active region  160  used in the second embodiment of the present invention has a single quantum well structure instead of a multi-quantum well structure. The quantum well layer  162  formed at the center of the active region  160  is 2-10 nm thick and is made of Ga x In 1-x N y As 1-y  where x and y are greater than 0 and less than 1. In the second embodiment, x is 0.65 and y is 0.01.  
         [0051]     Meanwhile, the first lower barrier layer  164 A and the first upper barrier layer  164 B are made of In x Ga 1-x As where x is greater than 0 and less than 1 to a thickness of 0.1-50 nm. In the second embodiment of the present invention, x is 0.35.  
         [0052]     Additionally, the second lower barrier layer  166 A and the second upper barrier layer  166 B are made of GaN x As 1-x , where x is greater than 0 and less than 1, to a thickness of 0.1-50 nm. In the second embodiment of the present invention, x is 0.02.  
         [0053]      FIGS. 5 and 6  are a cross-sectional view and an energy band diagram, respectively, of a quantum well structure according to a third embodiment of the present invention. An active region  170  used in the third embodiment of the present invention includes a first barrier layer  176  and a second barrier layer  178 , which are not symmetrical with respect to a quantum well layer  174 , as shown in  FIG. 5 . The quantum well layer  174  is made of Ga x In 1-x N y As 1-y , where x and y are greater than 0 and less than 1, to a thickness of 2-10 nm. In the third embodiment of the present invention, x is 0.65 and y is 0.01.  
         [0054]     According to the third embodiment, an auxiliary barrier  172  is formed of GaAs under the quantum well layer  174  to a thickness of 0-500 nm.  
         [0055]     Meanwhile, the first barrier layer  176  is 0.1-50 nm thick and is made of In x Ga 1-x As, where x is greater than 0 and less than 1. In the third embodiment of the present invention, x is 0.35.  
         [0056]     Additionally, the second barrier layer  178  is formed of GaN x As 1-x , where x is greater than 0 and less than 1, only on the first upper barrier  176  to a thickness of 0.1-20 nm. In the third embodiment of the present invention, x is 0.02.  
         [0057]      FIGS. 7 and 8  are a cross-sectional view and an energy band diagram, respectively, of a quantum well structure according to a fourth embodiment of the present invention. A quantum well layer  184  of an active region  180  in an edge-emitting semiconductor laser according to the fourth embodiment of the present invention is made of Ga x In 1-x N y As 1-y , where x and y are greater than 0 and less than 1, to a thickness of 2-10 nm. In the fourth embodiment, x is 0.65 and y is 0.01.  
         [0058]     Meanwhile, the first lower barrier layer  186 A and the first upper barrier layer  186 B are made of In x Ga 1-x As, where x is greater than 0 and less than 1, to a thickness of 0.1-50 nm. In the fourth embodiment of the present invention, x is 0.02.  
         [0059]     Additionally, the second lower barrier layer  182  is made of GaAs to a thickness of 0-500 nm. The second upper barrier layer  188  is made of GaN x As 1-x , where x is greater than 0 and less than 1, to a thickness of 0.1-20 nm. In the fourth embodiment of the present invention, x is 0.02.  
         [0060]     The fourth embodiment of the present invention differs from the first embodiment in that the composition and thickness of the first lower barrier layer  186 A and the first upper barrier layer  186 B are varied to induce compressive strain to the quantum well layer  184  but only the second upper barrier layer  188  is used to induce tensile strain to the quantum well layer  184 .  
         [0061]      FIG. 9  is a cross-sectional view illustrating a vertical cavity surface emitting laser (VCSEL) according to a fifth embodiment of the present invention. As shown in  FIG. 9 , a vertical cavity surface emitting laser  200  according to another embodiment of the present invention includes a semiconductor substrate  204 , an n-type electrode  202  formed on a lower surface of the semiconductor substrate  204 , an n-type distributed Bragg reflector (DBR) layer  240  formed on an upper surface of the semiconductor substrate  204 , an active region  210  formed on the n-type DBR layer  240 , a p-type DBR layer  230  formed on the active region  210 , a contact layer  220  formed on the p-type DBR layer  230 , and a p-type electrode  226  formed on the contact layer  220 .  
         [0062]     In addition, as shown in  FIG. 10 , the active region  210  includes a central barrier layer  212 ; a second lower barrier layer  218 A, a first lower barrier layer  216 A, and a lower quantum well layer  214 A, which are sequentially formed between the central barrier layer  212  and the n-type DBR layer  240 ; and an upper quantum well layer  214 B, a first upper barrier layer  216 B, and a second upper barrier layer  218 B, which are sequentially formed between the central barrier layer  212  and the p-type DBR layer  230 .  
         [0063]     According to the fifth embodiment of the present invention, the semiconductor substrate  204  is made of an n-type GaAs-based semiconductor material. The n-type BR layer  240  is formed by alternating a plurality of GaAs layers  242  and a plurality of AlGaAs layers  244 . The p-type DBR layer  230  is formed by alternating stacking a plurality of GaAs layers  232  and a plurality of AlGaAs layers  234 .  
         [0064]     The contact layer  220  is made of a p-type material, for example, GaAs, to a thickness of 800 Å. The n-type electrode  202  is made of AuGe, and the p-type electrode  226  is made of Ti.  
         [0065]     The VCSEL  200  according to the fifth embodiment of the present invention is a striped type. In order to allow a current to be applied to striped regions of the active region  210  from the p-type electrode  226 , an insulating layer  224  is formed of SiO 2  on the contact layer  220  and patterned into stripes.  
         [0066]     Although not illustrated in the drawings, in order to improve an ohmic contact between the p-type electrode  226  and the p-type contact layer  220 , a metal contact layer formed of Ti or Pt or as a stack of Ti and Pt layers may be further formed. Also, in order to also improve an ohmic contact between the n-type electrode  202  and the semiconductor substrate  204 , a metal contact layer formed of Ni or Au or as a stack of Ni and Au layers may be further included.  
         [0067]     The active region  210  shown in  FIG. 10  has the same structure and function as the active region  110  according to the first embodiment of the present invention.  
         [0068]     Although the VCSEL according to the fifth embodiment of the present invention is described in connection with the active region  210 , a VCSEL may be implemented using any one of the active regions according to the second through fourth embodiments described above.  
         [0069]      FIG. 11  is a graph of an emission wavelength versus barrier layer thickness in a quantum well structure according to the present invention, which includes a first barrier layer made of InGaAs and a second barrier layer made of GaNAs, when the thickness of the first barrier layer is fixed and the thickness of the second barrier layer is varied. The emission wavelength was measured using photoluminescence (PL) at room temperature. As is apparent from the graph of  FIG. 11 , the emission wavelength emitted from the quantum well is shifted toward a red wavelength range as the thickness of the GaNAs layer is reduced. Comparing to a conventional semiconductor laser including a GaAs barrier layer in a GaInNAs quantum well structure, red-shifting up to about 25 nm has occurred.  
         [0070]      FIG. 12  is a graph illustrating change of emission wavelength in a quantum well when the thickness of the second barrier layer made of GaNAs is fixed and the thickness of the first barrier layer made of InGaAs is varied. As is apparent from the graph of  FIG. 12 , the emission wavelength is shifted toward a longer wavelength range, up to 60 nm, as the thickness of the InGaAs layer is reduced.  
         [0071]      FIG. 13  is a graph of an emission wavelength in quantum well versus the amount of indium in an InGaAs layer. The graph of  FIG. 13  was experimentally obtained using a structure including a central barrier layer made of Ga 0.015 As 0.985  and first (GaNAs) and second (InGaAs) barrier layers, which have fixed thicknesses. As is apparent from  FIG. 13 , the emission wavelength becomes shortest when 20% of In is used.  
         [0072]      FIG. 14  is a graph of an emission wavelength in quantum well versus the amount of nitrogen (N) in the second barrier layer made of GaNAs. The graph of  FIG. 14  was experimentally obtained using a structure including a first barrier layer made of In 0.35 Ga 0.65 As, in which the first (InGaAs) and second (GaNAs) barrier layers have fixed thicknesses, while varying a DMHY flow rate.  
         [0073]     As described above, according to the present invention, by forming a plurality of barrier layers in a quantum well structure and by adjusting the thickness and composition of each of the barrier layers, a problem of optical quality degradation in a long wavelength range, which arises with conventional quantum well structures, can be solved.  
         [0074]     According to the present invention, emission wavelength shifting to a shorter wavelength range, which occurs when a GaInNAs quantum well structure is thermally treated, can be prevented.  
         [0075]     According to the present invention, using a GaAs-based quantum well structure, an emission wavelength of 1.3 μm or longer can be easily generated.  
         [0076]     According to the present invention, use of the first barrier layer made of InGaAs layer to induce compressive strain to the quantum well structure is advantageous in terms of optical gain.  
         [0077]     According to the present invention, long-wavelength emission can be economically achieved using a small amount of nitrogen MO source.  
         [0078]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.